Model Reduction for Keeping Gain Margin and Phase Margin Unchanged

The effects of model reduction on system characteristics, such as gain margin, phase margin, and bandwidth are investigated. A practical method is proposed so that the reduced models can approximate the frequency response of the original transfer function not only at S = 0 and S = ¿ but also at some specific points on the frequency response curve of the original system; thus the gain margin and/or phase margin can be kept unchanged. Examples are given, and comparisons with the methods given in current literature are made.