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This paper proposes a novel active fault tolerant control (FTC) scheme for a 3-degree-of-freedom (3-DOF) helicopter with sensor faults. As a challenge, only attitude angles are considered available, so that when the sensors measuring the elevation/travel angles are faulty, the system with respect to the remaining healthy outputs is not detectable. To circumvent this issue, a new interval observer (IO) with adaptive parameters is formulated, providing good estimates of both disturbances and unmeasurable states. This IO acts not only as a state estimator for nominal controller but also as a fault detection and isolation (FDI) observer for the fault occurrence and location. After the fault location is determined, two different fault estimation (FE) schemes are developed according to whether or not the system is detectable. Using the fault estimates, a fault tolerant controller is constructed to ensure the acceptable performance of the faulty system. Finally, experiments on the 3-DOF helicopter platform are conducted to verify the effectiveness of the proposed scheme.

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