George Stephanopoulos

Perspectives on the Synthesis of Plant-Wide Control Structures

The synthesis of plant-wide control structures has resurfaced as the most important design problem in process control. In this lecture we will provide a comparative analysis of various approaches, with an emphasis on how well they address the inherent theoretical and practical issues associated with the design of such control systems. Starting with a formal statement of the problem, which corresponds to a multi-objective optimization problem, we will argue that the central issue to be resolved is the translation of implicit operating objectives to sets of feedback-controlled variables.

The principle of the Optmizing Feedback Control Structures proposed as the formal medium for the identification of controlled variables. Furthermore, it is shown that the selection of the best sets of input (manipulated) and output (measured) variables for the formation of the controllers' structures is governed by classical control-theoretical aspects, such as open-loop gains, model uncertainity, and non-minimum phase process characteristics, Hierarchical viewing of a plant is posed as an effective mechanism to contain the complexities of the problem by streamlining the (i) specification of control objectives at different time-scales, (ii) modeling needs and model uncertainities, (iii) selection of measured and manipulated variables, and (iv) formation of the control structures.

In addition, we will discuss the characteristics of a phonomena-based, computer aided modeling environment, entitled MODELA, which is providing the hierarchical representation of the plants for the synthesis of plant-wide control structures. The overall approach will be illustrated on the synthesis of control structures for two continuous chemical plants.

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