Implementation of this method is very common. PID Self Tuner by Siemens (Fig. 1), PIDE by Rockwell Automation, TUNE by ABB and Profit PID by Honeywell are based on the step response tuning method. Other PLC producers have also their own implementation of the step response tuning method (B&R, Omron, GE Fanuc, Mitsubishi etc.). The relay method comes out from the critical gain and the critical period of the controlled system. There are two modes: a setting mode and a control mode. The relay regulator is connected in the closed loop in the setting phase. The process value oscillates along the set point in closed loop with the relay. The controlled systems behaviour from the frequency point of view is generally a lowpass filter. This fact is used for approximate analysis of the controlled system. The PID regulator constants are calculated from the identified critical gain and the critical period of the system by some control law, e.g. Ziegler-Nichols, Astrom-Hagglund, ChienHrones-Reswick or Coohen-Coon method.