Classical control revisited: Variations on a theme

Many powerful tools for automated controller synthesis now exist. However, classical control concepts are still very useful, because they often allow the designer to more readily employ engineering knowledge when making design trade-offs. This paper presents some recently developed compensator structures that are useful for loop shaping. The complex Proportional- Integral-Lead compensator is a variation of the cascade of of PI compensator and a lead compensator in which complex conjugate zeros provide additional design flexibility compared with the standard structure. Similarly the phase adjustable resonant compensator employs complex conjugate zeros to adjust the phase of at the resonant frequency making design with resonant compensators easier. The utility of these structures is demonstrated with a disk drive example.