A New QFT Design Method for SISO Cascaded-Loop Design

A new QFT design method for SISO cascaded-loop design, which is different from the conventional inner-outer procedure and the outer-inner procedure, is proposed in this paper. The desired uncertainty of the inner loop after closure is prescribed as a design specification before designing the inner controller, which is done by introducing an appropriate weight function. The inner loop design, therefore, is transferred into a sensitivity reduction problem. Then, the outer-loop design becomes that of a plant with mixed uncertainty which can be carried out as usual. In this method, the weight function is an important design factor, which actually allocates the control burden between the inner and outer controllers. This method not only removes the arbitrariness in the closure of the inner loop in the inner-outer procedure, but also can reduce the number of iterations needed, which therefore could give a significant reduction in the outer controller bandwidth as is demonstrated in two examples.