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Stability analysis in the frequency plane was developed by Ny quist and Bode. Design of the compensators were carried out by methods based on'trial and error Design rules such as Ziegler and Nichols are based on simple models, but they have succesfully been applied to pro cesses with a higher complexity. Root locus analysis was developed by W.R. Evans, and aims at obtaining prescribed dynam ical properties by determining a compensator, which provides a satisfactory closed loop pole placement Moreover,methods have been developed that provide explicit formulae for a compensator Methods, that aim at satisfying performance specifications formulated as conditions on the der. gtal of the square error, have been introduced by Newton, Gould Kaiser and further al Even though one of the ob jectives of feedb ack control is to to reduce the effects of model deviations, all these methods presume a perfect model, and only indirectly accounts for the fact, that a model can never be perfect Since several of the met hods have been applied succesfully for many years due to the inherent robustness of feedb ack compensators, it can be questioned whether it is reasonable to include model variations as an explicit design condition. There are, however, many examples where the introduction of feedback not automatically imply the robustness required to actual model riations. The following example originates from Lun891 Example 1.1(Robust Stability rocess is described by the model G(s)=1+718 which experimentally has been shown to prouide app rorimately the same step response as the real process. The process can, e.g., be controlled by a proportional compensator with the gain should be possible wi thout problems to select the gain K arbitrarily large It turns out in reality, however, that a more accurate model of the process is given by (1+718)(1+728) Now, it is apparent that the control system will become uns table, if k is chosen too large. To ensure stability, K must be chosen below the bound If for example To T2=0.1- T1, K must satisfy K< ll to ensure stability lead to a model that approximately describe the process, such a model can not be used for compensator design regie of validity of the model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