Cascade design of single input single output systems using H#8734; and quantitative feedback theory methodologies

摘要

This thesis considers the design of cascaded SISO control systems using the H∞and QFT methodologies. In the first part of the thesis the actual advantages offered bySingle Input Single Output (SISO) cascade loop structures are studied. In QuantitativeFeedback Theory(QFT) it is emphasized that the use of cascaded loops is primarily forthe reduction of bandwidth of the controllers. This in turn helps in considerablereduction of the adverse effects of high frequency noise. The question that arises then iswhether or not there are any substantial benefits to be gained by cascade loop design inthe low frequencies. It is shown using QFT methodology that there aren?t any advantagesgained in the low frequencies with the use of cascaded design. In effect it is concludedthat if the design is properly executed a single loop controller closed from the output tothe input will be sufficient to meet the typical performance specifications. This is shownusing an example where the mold level of a continuous casting process is to becontrolled. The plant being used has considerable uncertainty so that features of robustcontrol can be highlighted.In the second part the Robust Outer Loop bounds were generated analytically andexamined for certain properties. It was compared to the bounds generated by alreadyexisting algorithms.In the third part the inner outer QFT design was modified with the inner loopbeing designed using H∞ with the concept of sensitivity shaping. This design was verysimilar to the pure QFT design with the added advantage of having some automation.In the fourth part the H∞ methodology was used to design a two loop controlstructure. The idea was to compare this design to the QFT design. It was seen that H∞generated redundant controllers and pre filters.