The Nyquist plot of the sensor actuator open-loop FRF obtained theoretically (a, c) and experimentally (b, d), with (c, d) and without (a, b) low-pass filter included in the feedback loop. Clockwise rotation of the contour because of the additional low-pass filter is evident. Phase margins are as follows: (a) limit of stability, φ₁₁ = 0; (c) φ₂₁ = 14°; (b) φ₁₂ = 5.95°; (d) φ₂₂ = 10.91°.