Stability margins of multivariable control systems using Riemann surfaces

Riemann surfaces generalize control systems´ frequency response and root locus plots. It is shown how to determine gain and phase margins of square multivariable control systems directly over several representations of Riemann surfaces. It is also shown that the stability margins can be equally determined as a function of the negative or positive feedback gain applied to the system. Graphic representations are presented showing the stability margins as a function of the feedback gain, for both negative and positive feedback cases