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Assessment and Future Directions of Nonlinear Model Predictive ...

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492 R. Lepore et al.0.950.9w P;30.850.80.750.70 10 20 30 40 50 60 70 80 90 1000.550.5w M;20.450.40.350.30 10 20 30 40 50 60 70 80 90 100time (min)Fig. 5. Parametric sensitivity - Time evolution <strong>of</strong> the controlled variables; solid: nominalcase, dashed: mismatch in the fragmentation rate, dotted: mismatch in the transportvelocity0.920.370.910.360.90.35w P;30.890.88w M;20.340.330.870.320.860.310.850 20 40 60 80 1000.30 20 40 60 80 100100458040Reg (%)60q C(t/h)35304025200 20 40 60 80 100time (min)200 20 40 60 80 100time (min)Fig. 6. Plant-model mismatch: solid: without compensation, dotted: with a DMC-likecompensationor grindability) have a larger impact on the model prediction than the materialtransportation parameters.The performance <strong>of</strong> the control scheme is evaluated when a prediction modelwith erroneous fragmentation rates is used (which is the worst case <strong>of</strong> plant-modelmismatch). Figure 6 shows results corresponding to −5% errors in the fragmentationrates. Clearly, performance deteriorates <strong>and</strong> a significant steady-state errorappears. To alleviate this problem, a DMC-like compensation is proposed, whichconsiders the plant-model mismatch as a constant output disturbance ˆd k+i = ˆd kover the prediction horizon. An estimate <strong>of</strong> the disturbance ˆd k is obtained fromthe process output y k <strong>and</strong> the observer output, noted ȳ k , as follows:ˆd k = y k − ȳ k (6)

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