1 4.1 Traditional Design Methods 2 4.2 H∞ PID Controllers for the First-Order Plant 3 4.3 H∞ PID controller and the Smith Predictor 4 4.4 Quantitative Performance and Robustness 5 4.5 H∞ PID Controllers for the Second-Order Plant 6 4.6 All Stabilizing PID Controllers for Stable Plants