This paper addresses the problem of evaluation of alternative control configurations on the basis of structural characteristics of the process. Relative order is proposed as the main analysis tool for this purpose. Using tools from graph theory, it is shown that generic calculation of relative orders requires only structural information about the process. Relative order is interpreted as a structural measure of the initial sluggishness of the response, as well as a structural analog of dead time, which expresses fundamental structural limitations in the control quality. A matrix of relative orders of input/output pairs is introduced, which leads to a characterization of structural coupling among input and output process variables. On the basis of the above properties, general structural evaluation guidelines are proposed for alternative sets of manipulated inputs and alternative input/output pairs. The application of the theory is illustrated in the case of an evaporation unit, a chemical reactor and a network of heat exchangers.
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Acknowledgement-Financial support from the National Science Foundation through the grant CTS-8912836 is gratefully acknowledged.