A review of quantitative feedback theory as a robust control system design technique

• Published in: Transactions of the Institute of Measurement and Control 1992, Vol.14 (5), p.265-279
• Main Authors: Azvine, B., Wynne, R.J.
• Format: Article
• Language: English
• Subjects: Control systems; Control systems design; Control theory; Design engineering; Feedback; Mathematical models; Quantitative feedback theory; Robust control; Systems design
• ISSN: 0142-3312; 1477-0369
• DOI: 10.1177/014233129201400507

Performance robustness has been an important issue in control and it has been increasingly recognised as an area of significance in many design applications. This issue has particular importance among the engineering community because it offers a solution to control system design problems for plant and processes that cannot be described by a single linear time-invariant model. The aim of this paper is to describe one of the methods available for robust control system design called Quantitative Feedback Theory (QFT) and discuss its application to engineering problems. QFT is a powerful design technique based on multi degree-of-freedom use of feedback. The basic building block of a multi-degree-of-freedom system is a two-degree-of-freedom system in which two controllers are present, one inside the feedback loop, used to reduce closed-loop uncertainty, and the other prior to the loop, used to shape the input in order to achieve the required output. To show the power of the method, the application to an aero engine described by a non-linear model, linearised at five operating conditions, is discussed. A comparison of the QFT method is also made with the H-infinity method. It is shown that the H-infinity formulation of the QFT problem can produce a more conservative solution than the QFT method.