Two-degree-of-Freedom Robust PID Controller Design for Unstable System Using A Convex-Concave Optimization

The paper discusses a two-degree-of-freedom controller design based on a convex-concave optimization technique. The improvement of the reference tracking performance of an unstable system is considered. Combining both feedback and feedforward fixed-structure controller is preferable for practical work. The optimization constraints are functions in a frequency domain for the feedback control, and the bounded value constraints of the feedforward part are done in time-domain absolute values. The proposed method is tested with an unstable magnetic levitation system. The result shows the two-degree-of-freedom controller can improve the transient response significantly.