QFT fractional-order controllers for unstable plants with only one unstable pole

This paper presents the design of a robust fractional order controller for unstable plants with only one unstable pole based on quantitative feedback theory (QFT) using Particle Swarm Optimization (PSO). In this work, a fractional-order compensator, with a flexible controller structure, is introduced into the QFT design to give a better approximation of optimum open loop in Nichols. The fractional order controller is designed for not only minimum phase system but also uncertain non-minimum phase and unstable plants in QFT. It has been demonstrated that the fractional order controller can provide smaller high frequency gain than the integer order controller due to its extra tunable parameters.