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

Abstract

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-offreedom 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.

Robust Decentralized Load Frequency Control Using Multi Variable QFT Method in Deregulated Power Systems

“…It has also been extended to the nonlinear and time-varying cases. QFT often results in simple controllers which are easy to implement [18][19][20][21]. In this paper the goals are control of frequency and inter area tie-power with good damping of oscillation, also obtaining a good performance in all operating conditions and various loads and finally designing a low-order controller for easy implementation.…”

“…Output responses are acceptable if the magnitude of the output to be below the limits given by disturbance rejection models. Based on desired performance specification, we can obtain disturbance rejections bounds according to QFT method [18][19][20][21] . In this case, because tracking bounds are not considered, therefore, the disturbance rejection bounds or B D (jω i ) were considered as composite bounds B o (jω).…”

QFT Based Robust Positioning Control of the PMSM Using Automatic Loop Shaping with Teaching Learning Optimization

“…In the 1960s Issac Horowitz extended Bode's famous gainphase integrals and introduced quantitative feedback theory (QFT), a frequency domain controller design technique. QFT is based on the concept that only feedback is required for making a system robust and it must be shaped such that a set of objectives of tracking, disturbance rejection, robust stability, and so on are satisfied and these objectives have to be declared on the onset of design procedure [46]. QFT uses two-degree of freedom (2-DoF) control configuration, in which one controller ( ) is in the feedback loop and is meant for reducing the closed loop uncertainties and the other controller is the prefilter ( ) which is present prior to the loop in feed-forward configuration which is meant for shaping the input so that the desired output is obtained.…”

Comparison of different robust control methods in design of decentralized UPFC controllers

“…Based on the QFT technique [20][21][22][23]28], first step in the design process is to plot the plant uncertainties in Nichols diagram. This diagram is known as the system templates.…”

“…They have been successfully applied to design of both SISO and MIMO systems. Their applications have also been extended to non-linear and time-varying cases [19][20][21][22][23][24][25].…”