Approximate model techniques in robust control

The general single-input single-output control design problem is to design a robust controller to stabilise a plant. This plant is often highly complex or unknown therefore the problem considered in this paper is to design a controller based on an approximate plant. It is required that this controller will stabilise the original plant. The approximate feedback system is shown. Robust design is dependent on the availability of a suitable modelling error measure between the plant and the approximate model. The modelling error considered is E(t )=y(t)-y_A(t) where y(t) and y_A(t) are the unit step responses of the plant and approximate model respectively. A typical robust control criterion using an upper bound on this modelling error is given