I. Introduction
Quadrotor is a widely used UAV, since it is easy to develop. However, like any flying vehicle, a certification is mandatory to meet safety rules as far as failures and catastrophic effects are concerned. Therefore the fault tolerant control is of utmost importance in a quadrotor. The Fault-Tolerant Control System (FTCS) has the capacity to adapt itself to component faults automatically. In general, the FTCS can be classified into two types: Passive Fault-Tolerant Control Systems (PFTCS) and Active Fault-Tolerant Control Systems (AFTCS) [1]. In PFTCS, controllers are fixed and are designed to be robust against a class of foreseeable faults. These systems have limited fault-tolerant capabilities. In contrast to PFTCS, AFTCS react actively to system component failures by reconfiguring control actions so that the stability and acceptable performance of the whole system can be maintained. In this paper, we will focus on AFTCS, which include the Faults Detection and Diagnosis (FDD) and controller reconfiguration. A typical AFTCS structure is shown in Fig. 1.