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Fault diagnosis of complex control systems is one of the most important research topics among control engineering community during the last half century. In recent years, attention has tended towards designing of robust fault detection and diagnosis approaches both for linear and nonlinear systems. Many of the proposed methods in this field are based on robust observers which can efficiently estimate system states, in the presence of a wide class of modeling uncertainty and external disturbances. It should be noted that robust state estimation needs special conditions, which will limit its implementation. For nonlinear systems complexity is higher.} 
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