Robustness of decoupling with almost redundant inputs

Control design for an ill-conditioned plant is known to be problematic. One reason a plant may be ill-conditioned is that its control inputs may be almost redundant, in the sense that they have an almost identical effect upon the controlled outputs. In this paper we study robustness problems that may occur when a controller that decouples, or partially decouples, the plant is used with a plant that has almost redundant inputs.