Robust performance assessment of feedback control systems

The proper measure of closed-loop performance variation in the presence of modelplant mismatch is discussed in this paper. A generalized closed-loop error transfer function, which is a special representation of the dual Youla parameter and has a close relationship with the pointwise ν -gap metric, is proposed as the suitable means of representing closedloop performance variation in case of plant perturbation, and the closed loop performance variation measure is accordingly defined as its maximum singular value frequency by frequency. It is shown that this measure is essential and informative in characterizing closedloop performance variation. This measure is also shown to be readily applicable to on-line closed loop performance assessment or monitoring, even without the explicit model of the plant. Its variant, defined as the η -function, which features the relative performance variation as well as generalized stability margin variation with respect to the nominal plant, is also discussed.