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Figure 25 The controller-output varied depending on the estimate of the dinitrogen gas production. The figure is divided into sections A-F in which the controller behaved differently. To properly analyse the behaviour of the controllers the N 2-estimate was needed. The controllers N 2-estimate had not been recorded, instead this estimate was calculated in the same way the controller calculated it, from the stored on-line data. The estimate was normalised and plotted together with a normalised version of the DO level (Fig. 25) in order to compare the controllers behaviour to the N 2-estimate. The plot was divided into sections A-F by dotted red lines. The controller behaviour in each section could be predicted from the variation in the N 2-estimate (Note that only the variation of the estimate was of interest since the controller was independent of the absolute values. This allowed the analysis to be done with the normalised data.). In section A, C, and E the the N 2-estimate was falling independently of the changes in DO. This caused the controller to “lock� and only change between two different values. This behaviour had previously been predicted by the simulations for this exact situation. For section A and E the ammonium-meter was giving a varying signal, however, it was likely that the data was erroneous. Still, the variation in the N 2-estimate in these two sections was dominated by the NH +4 - N-variation. Since the ammonium was going up (see Figure 23) - seemingly unaffected by the changes in DO level - the N 2-estimate was going down. In section C the falling N 2-estimate was caused by a falling pH value (see Figure 24). Section B was characterised by an almost constant N 2-estimate. This made the behaviour of the controller somewhat unpredictable since small variations in either direction could have effects on the DO level. In the short section marked, D, the ammonium-meter seemingly started to work again (see Figure 23). The hole in the perforated membrane might have been temporarily covered. The quick drop in the measurements of NH +4 - N made the N 2-estimate go up. This caused the controller to start decreasing the DO level. The reason it was decreasing (as opposed to increasing) was that the last change the controller had done before the ammonium-meter came to life was in that direction. 39

/Olle_Trollberg  

http://www.sjostadsverket.se/download/18.50a499dd132037d524e80007759/Olle_Trollberg.pdf

/Olle_Trollberg  

http://www.sjostadsverket.se/download/18.50a499dd132037d524e80007759/Olle_Trollberg.pdf

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