These gain and phase margin curves are obtained using an exact calculation involving μ-
analysis. For computational efficiency, however, the tuning algorithm uses an approximate
calculation that can yield smaller margins in parts of the frequency range. To see the
lower bound used by the tuner, right-click on the plot, and select Systems > Tuned
Lower Bound.
If there is a significant gap between the true margins and the tuner approximation, try
increasing the D-scaling order. The default order is zero (static scaling). For tuning in
Control System Tuner, set the D-scaling order in the Margins Goal dialog box. For
command-line tuning, set this value using the ScalingOrder property of
TuningGoal.Margins.
Stability Margins in Control System Tuning