A multiple controller structure consisting of a typical feedback controller and an additive redundant controller is proposed for enhancing reliability of the control system. This scheme ensures the performance of tracking a given set-point input under failure in one of controllers or parameter perturbations of the plant. Dynamic reliability for the double control system is formulated in terms of the mean time to failure (MTTF) and asymptotic convergence of tracking error to zero is shown. For the case that the main controller is chosen as the optimal set-point racking controller and the redundant controller as the minimum prediction error adaptive controller or the model reference adaptive controller (MRAC) whose reference model is the closed-loop combination of the plant and the main controller, it is proven that the control system is more reliable than the system only with the single main controller in the sense of extended dynamic reliability, and further reliable operation of the control system is illustrated by numerical examples for a second order linear plant.
