The linear models of industrial plants described by transfer functions fall into three categories: stable plant models, integrating plant models, and unstable plant models [6]. The unstable plant model is the general one, since the stable plant model and the integrating plant model can be viewed as the special cases of the unstable plant model. In this sense, it is of great theoretical significance to study the stabilization and control problem of unstable plants. When the plant involves time delay, the stabilization and control problem of unstable plants is very challenging. This problem has attracted significant research interest over the past decade and is still of high interest. In [3], the problem of stabilization and control of a class of unstable first-order plants with time delays was considered. The stability of the closed-loop system was analyzed. The main result is an observer based scheme used to track step reference signals and to reject step disturbances. The most interesting feature of the scheme is that it can be used for unstable first-order plants with relatively large time delays. As the price, complex structures were used. The purpose of this discussion is to point out some aspects that can help the reader to better understand the result of the paper, and to provide a supplemental introduction on the key problem and the research development of the topic.
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