Robust stability in a classical and modern context

Excluding the open loop gain of a feedback system from a region about the origin of the Nichols chart yields a natural guarantee of robust stability which is commonly used in the analysis of flight control systems. This paper shows that a generalised form of M-circle, based upon a contour of constant vertical bar S vertical bar + vertical bar T vertical bar, is symmetrical on the Nichols chart and yields a stability margin consistent with the gain and phase margin requirements of the USAF. This new margin, motivated on purely classical grounds, is shown to be equal to the robust stability margin associated wit the loopshaping design procedure of McFarlane and Glover. A corresponding interpretation is obtained for the multivariable case.