A very important (if not the most important) problem in plantwide control is the issue of determining the control structure:
• Which “boxes” should we have and what information should be send between them?
Note that that we are here not interested in what should be inside the boxes, which is the
controller design or tuning problem. More precisely, control structure design is defined
as the structural decisions involved in control system design, including the following tasks
((Foss, 1973); (Morari, 1982); (Skogestad and Postlethwaite, 1996))
1. Selection of controlled outputs