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The design of a control system is invariably based on an assumed nominal model of the plant to be controlled. After the usual shnplifications, such as linearization about an operating point, lumped parameter approximation etc., one ends up most often with a linear time invariant system described by a prescribed set of differential equations for the nominal model of the plant. At this point, various established design strategies such as the frequency domain methods of classical control, or the state space methods of optimal control theory can be applied to the nominal model to produce a feedback controller that yields closed loop slability and au acceptable output time response.

An important and fimdamental practical requirement to be satisfied by the controller is the invarlance of the property of closed loop stability, under perturbations, from a suitable class, of the nominal plant modeh A controller satisfying this …