A frequency domain approach to control structure synthesis

The traditional methods for the actuator and sensor selection based on LQG theory have found only limited use in process control. The main reason is that LQG theory offers a handy design tool but the numerical value of the quadratic performance index lacks practical significance. A new approach to control structure synthesis using a frequency domain description of the system is suggested here. The screening of alternate control structures is carried out in two stages. At the first stage the fundamental limitations to control quality, which are shown to be RHP transmission zeroes and time delays, are identified and preliminary control structure selection is made. In the second stage the performance of the remaining systems is compared when they are robust with respect to a prespecified model-plant mismatch. The intuitive appeal of this screening procedure should make it attractive for industrial applications. Some features of this new method are illustrated on a heat exchanger network and a distillation column.