Open-loop strategy

Table 9.7. Comparison of Nominal and Worst-case Algorithm Based Optimum Design of Operating Policies (Open Loop Strategy)...

Open loop strategy A simple control strategy is adopted operate the column at constant reflux ratio (r), reboiler heat duty (QR) and terminate when the reboiler liquid level falls below a threshold level (H,hreshoid) to avoid reboiler running dry. This gives three operating parameters to be chosen. 

It has to be stressed that the terminology active and passive does not refer to open- or closed-loop control strategies. All the methods presented in this section to control the heat generated are open-loop strategies. 

The open-loop strategy implies that each players control is only a function of time, Ui = Ui t). A feedback strategy implies that each players control is also a function of state variables, ui = Ui t Xi t) Xj(t)). As in the static games, NE is obtained as a fixed point of the best response mapping by simultaneously solving a system of first-order optimality conditions for the players. Recall that to find the optimal control we first need to form a Hamiltonian. If we were to solve two individual non-competitive optimization problems, the Hamiltonians would be Hi = fi XiQi, i = 1,2, where Xi t) is an adjoint multiplier. However, with two players we also have to account for the state variable of the opponent so that the Hamiltonian becomes... 

For the feedback equilibrium the Hamiltonian is the same as for the open-loop strategy. However, the necessary conditions are somewhat different ... 

Figure 6-49. PI plus open-loop recycle trip strategy.

The control strategy for the root-locus diagram shown in Figure 5.24 is called PIDD, because of the additional open-loop zero. The system is unstable between K = 0.17 and K = 1.06, but exhibits good transient response at A" = 10.2 on both complex loci. 

Most automatic control strategies are a combination of feedback (closed-loop) and feedforward (open-loop) control components. Feedback control maintains a desired process condition by measuring and comparing it to a set point and initiating corrective action based on the difference between the desired and the actual conditions. Feedforward control provides fast compensation to disturbances if its "model" is accurate. The main limitation of feedforward is our inability to prepare perfect process models or to make perfectly accurate measurements. 

Comparison of the results obtained with the open and closed loop strategies shows the importance of considering uncertainty systematically, as there is a... 

However, design constraints may limit our ability to exercise this strategy concerning fresh reactant makeup, An upstream process may establish the reactant feed flow sent to the plant. A downstream process may require on-demand production, which fixes the product flowrate from the plant. In these cases, the development of the control strategy becomes more complex because we must somehow adjust the setpoint of the dominant variable on the basis of the production rate that has been specified externally. We must balance production rate with what has been specified externally. This cannot be done in an open-loop sense, Feedback of information about actual internal plant conditions is required to determine the accumulation or depletion of the reactant components. This concept was nicely illustrated by the control strategy in Fig. 2.16, In that scheme we fixed externally the flow of fresh reactant A feed. Also, we used reactor residence time (via the effluent flowrate)... 

Delays may not be directly observable, but may need to be inferred. Depending on where in the feedback loop the delay occurs, different control algorithms are required to cope with the delays [25] dead time and time constants require an algorithm that makes it possible to predict when an action is needed before the need. Feedback delays generate requirements to predict when a prior control action has taken effect and when resources will be available again. Such requirements may impose the need for some type of open loop or feedforward strategy to cope with... 

After definition of the optimum operation policies, different strategies can be used for actual implementation at the polymerization plant. The simplest strategy is the implementation of u°P at the plant, without using any sort of feedback signal for x and/or y, for evaluation of the operation performance. In mathematical terms, one might define the implementation of open-loop control strategies in the form ... 

Figure 8.9 Schematic representation of the open-loop control strategy.

Open-loop control strategies were developed and implemented to allow for reduction of transition times during grade transitions in continuous high-impact styrene polymerizations [61]. Similar strategies were also used to control the MWDs in emulsion homopolymerizations and to control the copolymer composition and the MWDs simultaneously in non-hnear emulsion polymerizations [36,37,182]. 

As mentioned previously the studied fermentation process to produce ethanol presents challenges in its process dynamics that exhibit oscillatory behavior, which affect process productivity and sustainabhity. To address these challenges, this section introduces a new sustainable process control framework that combines the biomimetic control strategy detailed earlier and the GREENSCOPE sustainabihty assessment tool. In this case study, the controlled variable is the concentration of product, Cp (see objective function defined earlier), and the dilution rate, Djn, is chosen as the manipulated variable. GREENSCOPE is employed to evaluate the sustainability performance of the system in open-loop and closed-loop operations. The obtained GITEENSCOPE indicator scores provide information on whether the implementation of the biomimetic controller for the fermentation process enables a more efficient and sustainable process operation. 

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