Robustness Analysis of Frequency-Domain PI Controller Design Procedures

PI controllers for first-order time-delayed processes can be designed on the basis of a normalized model according to open-loop frequency-domain specifications. To this purpose, it is convenient to refer to the charts representing the loci of constant stability margins and cross-over frequencies in the plane of normalized parameters. It is shown how the designer can avail himself of these charts to easily evaluate the effects of changes in the process parameters and, thus, control system robustness. In particular, the situation corresponding to the least stability margin is identified together with the related cross-over frequency. The procedure is illustrated with the aid of representative examples.