PURPOSE: To obtain the characteristics of an on-site process system controlled by means of a self-tuning controller.

CONSTITUTION: The open loop response to a process variable to a step impressed upon the control output of a controller is analyzed. A primary system having a pure time delay is used for approximating the actual response of a high-order on-site process system. The entire dead time 200 and time constant of the approximation by the primary system are decided. The maximum inclination 208 of the response of the process variable to the step is stored together with its occurring time and assigned process variable. The stored information is used for approximating the entire dead time 200. The time constant is decided by using a secondary time constant or a convergent point 68 existing within a prescribed range between the actual response data and primary approximation.