In most conventional control systems, controllers are designed for error-free systems without considering the possibility of failure. In order to overcome these limitations, modern complex systems use soft controllers that have been developed with the ability to adjust errors and tolerances, in order to meet the reliability and performance requirements. A fault-tolerant system is the goal of a control system that can maintain the system so that it becomes desirable and stable and efficient. The goal of a fault-tolerant system is to preserve the ability of the system to achieve those performances as close as possible to those that were at the beginning of the operation of the automated system. The emergence of a fault-tolerant system is motivated by different goals of the system, which improves reliability and safety in industrial processes and during flight control and operation of nuclear power plants. After the fault is detected and isolated, all possible structures of the system that preserve the predetermined performance are analyzed while the highest reliabilities indicate the optimal structure.