2604.01214 Gain Margin and Phase Margin Provide Contradictory Stability Assessments for 6 of 20 Benchmark Control Systems: A Structured Singular Value Reconciliation
Classical stability margins---gain margin (GM) and phase margin (PM)---remain the primary robustness indicators taught in control engineering curricula and applied in industrial practice. Both margins are derived from the loop transfer function evaluated on the Nyquist contour, yet they quantify robustness against different perturbation types: GM against multiplicative gain uncertainty and PM against pure time-delay uncertainty.