Fragility and robustness of the control system

A novel concept of the fragility of a control system is presented, though there are some different understandings about the fragility. Fragility means that the stability margin of a system is lost during an optimal design. It is pointed out in the paper, that the fragility problem is not caused by the controller fragility, and the real cause of fragility is the design tools or criteria used in the optimal design. For example, in the H_∞ optimal design, the concepts of stability margin are totally absent during the design process. In MIMO system design, the singular values are used instead of the individual physical variables, so the robust stability problem of the individual loop may be ignored. This concept of fragility is different from the robustness against the parameter uncertainty of the controller. And it is helpful for understanding of the phenomenon that a well-designed system may not be stable in practice and cannot work. Therefore it is important for both theory and practice. Design examples are also presented, one of which is from the authors' own experience.