This paper finds the necessary and sufficient existence conditions of fault tolerant robust supervisor of discrete event dynamic systems(DEDS) with model uncertainty. It develops two cases, untimed and timed DEDS models. It is assumed that the model of a controlled system is not known exactly but it belongs to the set of some models of the system. A supervisor does not know a model which it controls. A robust supervisor guarantees fault tolerant behavior of the system with model uncertainty. As a case study, we show a workcell in a production line which consists of two gas metal arc welding (GMAW) robots and two conveyors. The system has two operating modes. It is assumed that a supervisor does not know the mode in which it operates. A fault tolerant robust supervisor is designed to assure fault tolerance of the system no matter what mode it may control.
