How is fault-tolerant control achieved in critical systems?

Fault-tolerant control maintains acceptable performance despite component failures. Key elements: fault detection and isolation (FDI) - detect when/where failure occurred using residuals, observers, or parameter estimation; fault accommodation - reconfigure controller for degraded plant (virtual actuator/sensor concepts, control reallocation, adaptive control). Redundancy levels include hardware redundancy (multiple actuators/sensors), analytical redundancy (estimate from model), and functional redundancy (use alternative control paths). Critical systems (aircraft, nuclear) require formal verification of fault tolerance.