Hierarchical control system design

The presence of three distinct time horizons in the process dynamics, as evinced by the analysis above, warrants the use of a hierarchical control structure that addresses the distributed (unit-level) and plant-wide control objectives separately. 

The first (distributed) layer of the control structure proposed in Section 5.4 was implemented as described in Equation (5.42), i.e., by stabilizing the holdups of the units within the recycle loop (the reactor and the vapor phase of the condenser) with proportional control laws. The liquid holdup in the condenser 

For controller-design purposes, it is practical to derive a state-space realization of the dynamics after the fastest boundary layer (Equation (5.20)) using a coordinate change (5.18) in which the control objectives appear directly. Thus, rather than expressing the dynamics of the system in terms of total holdups, we used 

Using the symbolic calculation engine available in Matlab ,2 we obtained the following description of the intermediate dynamics of the reactor-condenser process  

We then used the model (5.51) as the basis for synthesizing an input-output linearizing controller with integral action (Daoutidis and Kravaris 1992) for the product purity xB = 1 — Cs - C6, using the condenser vapor holdup setpoint 

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Section 2.2, in that two layers of control action involving separate controllers are proposed, whereas composite control relies on a single (possibly multivariable) controller with two components, a fast one and a slow one. Thus, the hierarchical control structure accounts for the separation of the flow rates of the process streams into two groups of inputs that act upon the dynamics in the different time scales. On the other hand, composite controller design (Figure 2.9) presupposes that the available manipulated inputs impact both the fast and the slow dynamics and relies on one set of inputs to regulate both components of the system dynamics. 

Remark 3.4. In the context of the present chapter (and of the remainder of the book), the term hierarchical control structure reflects the use of two (or multiple) coordinated tiers of control action, and should not be confused with hierarchical plant-wide controller design strategies (see, e.g. Ponton and Laing 1993, Luyben et al. 1997, Zheng et al. 1999, Antelo et al. 2007, Scattolini 2009, and references therein), which use the term hierarchy to denote a set of guidelines, to be followed in sequence, for designing the control system for a chemical plant. 

The previous chapters have concentrated on analyzing the material-balance dynamics of several classes of integrated process systems. We demonstrated that the dynamic behavior of the systems considered exhibits several time scales and described a method for the derivation of reduced-order models describing the dynamics in each time scale. Also, a hierarchical controller design framework was introduced, with distributed control of the fast dynamics and supervisory control of the dynamics at the systems level. 

The trade-offs among process design, optimization and control must be considered. The hierarchical or distributed nature of the plant or process may need to be exploited in an advanced control scheme. The operation of energy-integrated plants requires design of control systems which are decentralized (such as with microprocessors) but which respond to overall plant objectives via a communication link to a larger computer. 

Once a complete flowsheet has been developed, the operability and control of the process can be considered. Moreover, the economic incentive for modifying the flowsheet to improve the control can be considered. Then a (hierarchical) procedure for the synthesis of a control system for the complete plant can be used as an additional tool for screening the process alternatives, and a preliminary hazardous operations study can be initiated. The results of this conceptual design study then provides an estimate of the economic incentive for initiating a more rigorous design study. 

Douglas, J. M. Conceptual Design of Chemical Processes, New York McGraw-Hill11988,. Ponton. J. W., and Laing, D, M. A Hierarchical Approach to the Design of Process Control Systems, Cham.. Eng. Res. Des., 71, 181-188 (.1993). 

Ogunnaike, B., W. H. Ray, 1998, Modelling and Process Control, Academic Press Ponton, J. W., D. M. Liang, 1993, Hierarchical approach to the design of process control system, Chem. Eng. Res. Des., 70, 181 Seider, W. D., J. D. Seader, D. R. Lewin, 1999, Process Design Principles Synthesis, Analysis and Evaluation, Wiley... 

Accidents in STAMP are the result of a complex process that results in the system behavior violating the safety constraints. The safety constraints are enforced by the control loops between the various levels of the hierarchical control structure that are in place during design, development, manufacturing, and operations. 

The functional viewpoint has been adopted to design additional (lower) hierarchic levels. Examples of the lierarchic levels 3 and 4 for control systems are shown in Table 4,... 

Creating a richer model of causation. (Leveson and Dulac 2005) propose the STAMP accident model and the STPA hazard assessment approach. STAMP is based on systems-theoretic concepts of hierarchical control, internal models of the environment and a classification of control errors. STPA takes that classification as the basis for iterative integrated control system safety assessment. At each design iteration the design is assessed and constraints are derived (equivalent to derived safety requirements) and imposed on further design iterations. 

Puigjaner and co-workers in Chapter D2 explore the interactions between the various decisions levels linked to the batch control system. The work is motivated by the increasing shift in chemical industry to higher added value products that are usually produced batch-wise (Ref 1). Optimal design, analysis, and scheduling of batch processes lead to hierarchical and interconnected decision levels that require a holistic approach. A comprehensive overview of the requirements and standards for automatic batch control systems provides the basis for the... 

The variety of hierarchical design procedures for plantwide control systems that employed modular analysis (Ref. 19-20), decomposition policies (Ref. 21-22), interaction analysis (Ref. 17), lexicographical analysis (Ref. 23), and economic potential analysis (Ref 24) may lead to more than one control systems for the same plant configuration therefore, a further evaluation... 

A control structure obtained using the hierarchical procedures in the previous section normally can be expected to work reasonably well. However, the only valid test of that conjecture is actually to perform simulations or plant tests after individual controllers have been tuned. In that way, one can determine just how well the controlled system deals with disturbances, production rate changes, and so on. For our purposes, we have focused initially on the core process units in the plant (reactor, flash unit, and recycle tank) to determine how well a design likely would work if it were developed using heuristics, strongly guided by simplified structural analysis. Other credible alternatives are possible. Which of the many alternatives are acceptable... 

Figure 3. Hierarchical levels of metabolic control. Sites of metabolic control are designated as (1) plasma membrane level active transport systems, hormone receptors (2) cytoplasmic level hormone binding protein complex, signal molecule generation (3) enzymatic level steady-state enzymatic pathway, servomechanisms, enzyme degradation (4) ribosomal level protein biosynthesis (5) nuclear level hormonal control of gene action, operon control of gene action (substrate induction, product repression). The symbol,, indicates inhibition of a reaction.

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