Observations on PID control for unknown systems

PID control (Proportional-Integral-Derivative) has been widely researched and studied topic of automatic control for decades. While other, more sophisticated control methods have been introduced, PID control has still remained as a bread and butter control method for most control applications. This paper presents some fundamental, elegant and general observations on SISO (Single-Input-Single-Output) PID control without knowledge on the process model, or in some cases, with using only the minimum knowledge of the process model.