Controller reduction with closed loop H 1 1 performance constraints: a QFT perspective
GILLES FERRERES{{ and YANN LE GORREC{Assume that an initial stabilizing controller K 0 … s † , which satis® es various closed loop frequency domain speci® cations, has been a priori synthesized using, e.g. H1 control, · synthesis techniques or closed loop convex synthesis. Remembering that the order of K 0 … s † is typically at least equal to the order of the plant to be controlled, the aim of this paper is to ® nd a stabilizing reduced order controller, which also satis® es the performance speci® cations and which moreover minimizes the open loop bandwidth. To this aim, the structured singular value · is used to translate at each frequency closed loop frequency domain speci® cations into requirements on the frequency response of the controller. The principle of our reduction method is thus very close to the original idea of the SISO QFT design approach, except that the problem of translating closed loop frequency domain speci® cations into open loop ones is much more complex in the MIMO case. The problem reduces to the issue of ® nding a controller, whose frequency response belongs at each frequency to a template. The convexity of these templates greatly facilitates the practical realization of the controller. As a ® nal point, the method is successfully applied to a standard H1 problem, which is extracted from the · Analysis and Synthesis Toolbox of Matlab, namely the synthesis of an autopilot for the space shuttle.