Optimal restricted structure control with prespecified gain or phase margins

Restricted structure controllers have previously been defined, where the controller structure and order are predefined. These enable a cost-function to be minimised to obtain the best controller tuning parameters; however, the gain and phase margins were incidental. A constrained optimisation method is proposed that enables specified gain or phase-margins to be achieved. In fact, these robustness margins are generalised and the optimisation method enables them to be achieved exactly, but at the price of a cost increase.