Though PID control has a long history as much as its life force since Ziegler and Nichols published the empirical tuning rules in 1942, surprisingly, it has never been changed in the structure itself. The strength of PID control lies in the simplicity, lucid meaning, and clear e?ect. Though it must be a widely - cepted controller for mechanical control systems, it is still short of theoretical bases, e.g., optimality, performance tuning rules, automatic performance t- ing method, and output feedback PID control have not been clearly presented...