The implementation of high gain feedback laws however entails large (either in magnitude or in energy) and fast varying control inputs and hence large actuation capacities. Another intricate feature of high gain feedback design methods is the restriction it sometimes places on the open loop system in order to achieve certain design objectives. One of most common such restrictions is the minimum phase assumption, as demonstrated by the classical asymptotic root-loci design method.
Low gain feedback has been conceived to either avoid or to complement high gain feedback whenever such "unpleasant" features of high gain feedback prevent certain control objectives from being achieved. It also helps deal with nonlinearities other than saturation and numerical issues in robust control design.
The development of the low gain feedback design methodology, and the subsequent low-and-high gain design methodology, enhances the industrial relevance of modern control theory by providing solutions to a wide range of problems that are of paramount practical importance. This book gives a unified presentation of the low gain and low-and-high gain design methodologies.
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