Systematic nonlinear controller design approach based on quasilinear system models

Recent research in the area of describing function approaches (both random-input and sinusoidal-input) has laid the foundation for a systematic approach to designing controllers for nonlinear plants. An overview of one school of though in this area is presented, typified by the following key ideas: (1) quasilinear models of the nonlinear system that account for the operating range of system variables must be more realistic than conventional linear models; (2) a staged design approach that applies more power to difficult nonlinear plants than to easy ones is more sensible than a single design method, and (3) the designer should be led to a nonlinear controller design if and only if it is really required. These points are developed, illustrated and justified