An on-line self-tuning method of PI Controller was developed. The method tunes up the P and I gain according to the integral square error (ISE) of the control parameter. When the ISE is less than a pre-defined critical value, the P and I gains are kept unchanged. If the ISE is higher than the critical value, P and I gains are changed based on the low.
Frequency proportion (LFP) of ISE. The P and I gain are decreased when the LEP is less than a pre-defined value. Otherwise the P and I　 gain are increased properly . This method can be implemented in existing building automation systems. This paper presents the fundamental theory, method, and implementation examples.