Design of QFT fractional order robust controller for non-minimum phase and unstable plants

This paper presents the automatic loop shaping design of robust fractional order controllers for non-minimum phase or unstable plants with only one unstable pole based on quantitative feedback theory (QFT). Fractional-order controllers, with a flexible controller structure, are 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.