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 questions 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.