2.8 A new plantwide control design procedure
Based the above review and as a conclusion to this paper, we propose a plantwide design
procedure. The procedure mainly follows the mathematically oriented approach, but with
some elements from the process oriented approach. We propose to first perform a top-down
analysis to select controlled variables, based on the ideas of self-optimizing control. For this
we need a steady state model and operational objectives (steady state economics).The result
is one or more alternative sets for (primary) controlled variables ( y1 = c)
This top-down analysis is followed by a bottom-up assignment and possibly design of
the control loops. This is done in a sequential manner starting with stabilization. Next we
consider the fast loops needed for local disturbance rejection. Here we may make use of
(extra) secondary measurements (y2). This is the “regulatory” control layer (system). The
objective for the regulatory layer is that manual operation of the plant is possible after these
loops are closed.
We now have as degrees of freedom the setpoints of the regulatory layer (r2) plus any
unused manipulators (u2), these should be used to control the primary outputs (y1). This control layer is often called the supervisory control layer. This name may be misleading
or too limited. Often a supervisory controller is only “activated” when something has gone
wrong (e.g. logical switches), but we will also use it for “active” controllers. An alternative
name could have been advanced control layer, but advanced is a relative word and it is not
given that the controllers in this layer are advanced.
There are two main approaches here: Use of single loop (decentralized) controllers with possible feed-forward links, or use of multivariable control, e.g. decoupling or model predictive control (MPC). Properly designed multivariable controllers give better performance, but this must be traded against the cost of obtaining and maintaining the models used in the controller.
The main result of this will be the control structure, but controller tuning may also be obtained. Finally, nonlinear dynamic simulations should be performed to validate the proposed control structure.