ABSTRACT
The understanding of feedback control principles and the ability to implement them in real-world applications are among the main achievements of engineers in the last century. Moreover, the success of engineers controlling unstable plants that can hardly be operated manually opened the door to new opportunities. This chapter introduces the practical and comprehensive method proposed by the author to compute the Nyquist stability criterion directly in the Nichols chart. The method can be applied to linear time invariant (LTI) closed-loop systems with minimum and non-minimum phase zeros, stable and unstable poles, poles at the origin with diverse multiplicity and systems defined by nonrational functions, such as plants with time delay. The chapter shows how to apply the method with some illustrative examples. It provides guidelines to design controllers to stabilize unstable plants when dealing with frequency-domain control techniques, and in particular with the QFT robust control design methodology.
