Control structure selection: A geometric perspective

A structural perspective on the issue of control structure selection is discussed, using appropriate formulations of the concept of relative order. A number of attractive properties of the relative order are rigorously established: its generic calculation requires only structural information for the process; it provides an intuitively appealing measure of dynamic interaction, responsiveness, and structural coupling; and it poses fundamental limitations in the achievable control quality. On the basis of these properties, general guidelines are developed for the choice of a control structure with favorable input/output pairings and favorable servo and regulatory characteristics. The proposed approach has a purely structural character, and it does not substitute static gain or dominant time constant considerations or address limitations arising from open-loop instability and non-minimum-phase characteristics. It is applicable to both linear and nonlinear systems at the preliminary stages of the design procedure, allowing the designer to systematically evaluate alternative control structures on the basis of their structural characteristics.