Input-output decoupling by output feedback with disturbance attenuation of H _∞ control for any square system

The combined problem of diagonal decoupling by output feedback with disturbance attenuation performance for any square system is formulated. A sufficient condition which is more general than the necessary and sufficient one of static state feedback for the existence of the controller is determined. The necessary and sufficient conditions for the controller and decoupled system that are both stable are presented. An analytical form and a numerical computable method for the controller are obtained for the first time when the condition is satisfied. The correctness of our method is tested by simulations for an example.