A chemical plant may have thousands of measurements and control loops. By the term plantwide control is not meant the tuning and behavior of each of these loops, but rather the formulation of the overall control problem, and how to decompose the overall problem into smaller blocks, that is, selection of the structure of the control system (control structure design). 25 years ago Alan Foss challenged the process control research community in his paper Critique of chemical process control theory (AIChE J., 1973). He wrote:
The central issue to be resolved ... is the determination of control system structure. Which variables should be measured, which inputs should be manipulated and which links should be made between the two sets?And he added
There is more than a suspicion that the work of a genius is needed here, for without it the control configuration problem will likely remain in a primitive, hazily stated and wholly unmanageable form.May be this last statement has worked as an deterrent, because there has only been limited activity in this field over the last 25 years. Actually, the approach to plantwide control is still very much along the lines described by Page Buckley in his book from 1964.
Of course, the control field has made many advances over these years, for example, in methods for and applications of on-line optimization and predictive control. Advances has also been made in control theory and in the formulation of tools for analyzing the controllability of a plant. These latter tools can be most helpful in screening alternative control structures.
Maybe the most important reason for the slow progress in plantwide control theory is that most people do not realize that there is an issue. But ask the question: Why are we controlling hundreds of temperatures, pressures and compositions in a chemical plant, when there is no specification on most of these variables? Is it just because we can measure them or is there some deeper reason?
The concept of self-optimizing control seems to provide the answer to the above question, and the idea will be explained in more detail in the talk.