Classical Frequency-Domain Design Methods
The design of feedback control systems in industry is often carried out using frequency-response (FR) methods. Frequency-response design is used primarily because it provides good designs in the face of uncertainty in the plant model. For example, for systems with poorly known or changing high-frequency resonances, we can temper the feedback design to alleviate the effects of those uncertainties. This tempering can be carried out more easily using FR design without extensive modeling of the uncertainties. The method is most effective for systems that are stable in open loop; however, it can also be applied to systems with instabilities. This article will introduce the reader to methods of design using lead and lag compensation. It will also cover the use of FR design to reduce steady-state errors and to improve robustness to uncertainties in high-frequency dynamics.