A General Identification Approach

Abstract

Chapters 5 through 7 have covered in detail process frequency response estimation using relay feedback and conversion of frequency responses to transfer function models. The methods developed there are very useful in applications. There are, however, a few other issues which deserve attention. First, many industrial control systems already run in closed-loop before an identification test and/or controller retuning is carried out. In some cases, performing identification experiments under closed-loop conditions may be necessary for safety or economic reasons, or if the system contains inherent feedback mechanisms (Forssell and Ljung, 1999). It is thus desirable not to disconnect the existing controller during the test. This gives rise to the closed-loop identification problem, requiring that a test be conducted on a closed-loop system, but not on the process itself.