On direct versus indirect methods for reduced-order controller design

Comparisons are obtained between direct (parameter optimization) and indirect (controller reduction) methods in reduced-order controller design. In particular, the authors compare the controller canonical correlation coefficients method of C. De Villemagne and R.E. Skelton (1988) to the optimal projection theory of D.C. Hyland and D.S. Bernstein (1984). It is noted that suboptimal methods may, in certain cases, be simpler to implement than optimal methods. Thus, the choice of direct versus indirect methods can be viewed as a tradeoff between design simplicity and actual control-system performance