Robust performance with respect to block diagonal input uncertainty

The authors consider the robust performance problem of maintaining a small output sensitivity function despite the presence of block diagonal uncertainty at the plant input. For the case of two blocks of uncertainty, they study this problem by analyzing bounds on the structured singular value. It is shown that the potential difficulty of attaining robust performance is primarily determined by the difficulty of maintaining a small set of interaction parameters. The authors derive a robustness indicator which is useful in detecting this potential difficulty. The indicator is a function of both the plant condition number and its input directionality properties