Cascade Fractional-Order PI Control of a Linear Positioning System

Abstract

This paper proposes a method for designing robust fractional-order proportional-integral (FOPI) controllers to be employed in a cascade control system. The FOPI controllers are employed for controlling two nested loops. The design is based on performance and robustness specifications in the frequency domain. Taking inspiration from well-known tuning rules, the open-loop frequency response in the two nested loops is shaped around the gain crossover frequency to obtain a nearly flat phase diagram, then a nearly constant phase margin. The method is tested to control the speed and position of a linearly sliding motor.