  A method to quantify robust performance for situations where structured
parameter variations and initial state errors rather than extraneous
disturbances are the main performance limiting factors is presented. The
approach is based on the error dynamics, the difference between nominal and
perturbed dynamics, driven by either the unperturbed or perturbed state, rather
than an artificially imposed disturbance. The unperturbed versus perturbed
dichotomy can be interpreted as the relative error dynamics scaled by either
the unperturbed or perturbed dynamics. The error dynamics driven by unperturbed
state has the unique feature of decoupling the effect of physically meaningful
uncertainties from an additive disturbance. The perturbed case offers the
possibility to side-step Structured Singular Value (SSV) computations.
Applications to a lightly damped mechanical system and a slowly dephasing
quantum system demonstrate the usefulness of the concepts across a broad range
of systems. Finally, a fixed-point algorithm specifically developed for quantum
systems with state transitions depending in a nonlinear fashion on
uncertainties is proposed as the substitute for classical SSV.
