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A framework for active fault tolerant control against time-varying actuator fault is investigated, aiming to improve the robustness, sensitivity of fault detection and the rapidity of whole diagnosis and compensation procedure. A high-gain observer technique is extended to design a residual signal with the estimation error of system states and the derivatives of system output. Then, a compensator for actuator fault is directly constructed based on the fault information from the diagnosis procedure. A explicit relationship between the robustness, rapidity and sensitivity of the proposed fault diagnosis scheme with the observer parameters is strictly derived. By preselecting the observer/controller parameters, the set of detectable faults, the time of detection and compensation and the bound of the closed-system signals are quantified. The theoretical results are illustrated by a simulation example of surface-mounted permanent magnet synchronous motors.