A synthesized design of sliding-mode and Luenberger observers for early detection of incipient faults

In this paper, a new design of sliding mode observers (SMOs) for the purpose of early detection of incipient faults is presented. This work is motivated by the fact that the strong robustness of the conventional SMO to bounded disturbances inevitably makes it insensitive to incipient faults when they are small in size during the initial phase; as a result, the fault-detection residual generated through the SMO is not sensitive to incipient faults for a certain length of time until they have become severe. On the contrary, the new design proposed in this paper ensures that the overall system is sensitive to incipient faults while remaining robust to disturbances. The design presented in this paper is based on the transformation of the system into two subsystems. One of these subsystems is free from disturbances and/or modeling errors: for this subsystem, a Luenberger observer is constructed; the other subsystem is subject to disturbances: an SMO is designed for this subsystem so that the effect of the disturbances is eliminated. Together, the entire system achieves both robustness to disturbances and sensitivity to incipient faults. Stability and convergence of the proposed SMO and Lueneberger observer are proved, and an aircraft example is employed to demonstrate the effectiveness of the proposed design approach. Copyright (C) 2010 John Wiley & Sons, Ltd.