1. Design of inner cascade loops.
2. Design of basic decentralized loops, except those associated with quality and production
rate.
3. Production rate and quality controls.
4. Higher layer controls.
The decomposition in stages 1-3 is based on the speed of the loops. In stage 1 the idea is
to locally reduce the effect of disturbances. In stage 2 there generally are a large number of
alternative configurations. These may be screened using simple controllability tools, such
as the RGA. One problem of selecting outputs based on a controllability analysis is that one may end up with the outputs that are easy to control, rather than the ones that are important
to control. The method is applied to the Tennessee Eastman test problem.
Douglas (1988), at page 414, presents a hierarchy for control system design, based on
the dynamics that are involved. In this hierarchy the viewpoint is not on the flowsheet but
on steady-state, normal dynamic response and abnormal dynamic operation. Zheng et al.
(1999) continue this work. They places a greater attention to feasibility of the constraints and
robust optimality (self-optimizing control). In addition they proposes to use minimum surge
capacity as a dynamic cost, (Zheng and Mahajannam, 1999). They have not documented that
this will capture the true dynamic cost.