Most (if not all) available control theories assume that a control structure is given at the outset. They therefore fail to answer some basic questions that a control engineer regularly meets in practice (Foss 1973): Which variables should be controlled, which variables should be measured, which inputs should be manipulated, and which links should be made between them? These are the question that plantwide control tries to answer.
There are two main approaches to the problem, a mathematically oriented approach (control structure design) and a process oriented approach. Both approaches are reviewed in the paper. Emphasis is put on the selection of controlled variables, and it is shown that the idea of self-optimizing control provides a link between steady-state optimization and control.
We also provide some definitions of terms used within the area of plantwide control.