Control effort reduction in quantitative feedback theory for multivariable systems

Quantitative feedback theory (QFT) method for multi input multi output (MIMO) systems divides design problem into some single input single output (SISO) problems. In this method, interactions between loops are considered as disturbance signals. On the other hand, any increase in disturbance signals cause an increased control effort. In this paper, two new methods are presented which are used in QFT design for MIMO systems to reduce control effort. In these methods, the certain part of the non-diagonal elements of the transfer function is minimized as far as possible and MIMO QFT controller is designed for the obtained system. Reduction of certain parts is carried out by two different methods. In the first method, the system is decoupled by a state feedback in the nominal point, and, in the second method, the certain parts are reduced by LQR state feedback. The final system will be robust because its final feedback is in the form of QFT feedback. The results obtained from simulations reveal that control effort is reduced by suggested methods.