Alternative approaches in frequency domain design of single loop feedback systems with plant uncertainty

The frequency domain design of robust feedback control systems involves the fitting of a designable complex function of frequency to specification/uncertainty derived constraints. At present there are two basically different approaches to these problems: the several gain vs. frequency approaches and the two gain-phase vs. frequency approaches. Unfortunately, the commonalities, advantages and disadvantages of these approaches are not widely appreciated. In this paper the authors develop a common framework for examining these alternatives and use this framework to reveal some of their similarities, strengths and weaknesses.