Robust Control in Gap Metric

Robust control needs to start with a model of system uncertainty. What is a good uncertainty model? First it needs to capture the possible system perturbations and uncertainties. Second it needs to be mathematically tractable. The gap metric was introduced by Zames and El-Sakkary for this purpose. Its study climaxed in an award-winning paper by Georgiou and Smith. A modified gap, called the -gap, was later discovered by Vinnicombe and was shown to have advantages. With these metrics in hand, robust stabilization issues can be nicely addressed.