On Choice of the Nominal Plant in Quantitative Feedback Theory

An important statement in Quantitative Feedback Theory(QFT) is that the nominal plant can be chosen arbitrarily whether or not it is a member of the plant set. Based on this statement, a design method for non-minimum phase and unstable plants proposed by Horowitz is to choose the stable minimum phase plant as the nominal plant. This report shows this statement is incorrect by several counterexamples including non-minimum phase plants, unstable plants and even stable minimum phase plants.When the nominal case is chosen out with the plant set, an improved design procedure is presented. In addition, other existing design methods including Singular-G and linear programming are reviewed and the disadvantages and errors are pointed out.