AbstractThe proposed method Is appropriate to handle large linear systems consisting of interconnected subsystems and having a high system order (100 ... 1000 e.g.). After decomposition into subsystems, these are simplified by an aggregation technique. At the same time a local feedback on a lower level can be included. The aggregate and possibly locally controlled subsystems are then recomposed to remove the decomposition. The most essential topic of this paper is the answer how to simplify the subsystems such that recomposing the simplified ones approximates the original system behaviour sufficiently well. The method has proved to be robust enough to apply it repeatedly. This has also been done with the application example of a metal strip processing line (system order 230) that will be presented in some detail in the second part of this paper.