| The recent years have witnessed the extensive applications of multi-rotor unmanned aerial vehicle(UAV)in different military and civil applications including fighting against the enemy,aerial photography and express delivery because of its unique advantages,such as small size,vertical take-off and landing,flexible maneuvering and so on.The multi-rotor UAV can be divided into fixed angle multi-rotor UAV and variable angle multi-rotor UAV according to whether the actuator can be tilted.The fixed angle multi-rotor UAV usually consists of quadrotor UAV,hex-rotor UAV,eight-rotor UAV,coaxial six-rotor UAV and so on,whose dynamics analysis is quite similar.The tilt tri-rotor UAV is a typical variable angle multi-rotor UAV,which has the advantages of smaller size,lower energy consumption and better maneuverability.As a new configuration,the mechanical structure and torque allocation principle of the tilt tri-rotor UAV is different form that of the fixed angle multi-rotor UAV.This dissertation mainly takes the quadrotor UAV and the tilt tri-rotor UAV into account.To maintain the stable attitude control performance of the quadrotor UAV,the actuators(usually DC motors)and the propellers should response very fast,which increases their failure probability.While a tilt rear servo is always mounted on the tail of the trirotor UAV in order to maintain its torque balance.As a result,the continuous deflecting of the rear servo makes it the most vulnerable device of the tri-rotor UAV.Since the multi-rotor UAVs including quadrotor UAV and the tilt tri-rotor UAV are typical nonlinear systems which have the characteristics of open-loop instability,underactuated property and strong couplings,once the actuator’s fault happens,the flight performance of the UAV will drop rapidly or even out of control.How to ensure the UAV’s effective control under the circumstances of actuator’s fault is becoming a popular topic in UAV research field.To solve the control problem of the multi-rotor UAV after the actuator’s fault happens,the primary fault tolerant control(FTC)methods can be divided into two categories including active fault tolerant control(AFTC)and passive fault tolerant control(PFTC).The AFTC usually requires fault diagnosis,fault isolation and fault reconstruction steps,while PFTC depends on the robustness of the control algorithm to counteract the fault.The former one makes different decisions according to different problems so that it can produce good control performance,but it requires large amount of calculations and is difficult to implement in practice,and the current verification is mainly realized through numerical simulations.The latter one is of simpler structure,lower computational complexity,so that it has been widely used,but the control performance needs to be improved.Therefore,how to design efficient FTC methods remains a great challenge.Focusing on the limitations of the current UAV FTC methods,this dissertation mainly takes quadrotor UAV and tri-rotor UAV as research objects.FTC against partial loss of effectiveness or stuck fault of actuators will be considered in detail and the research contents are mainly composed by the following sections:1)The coordinate system utilized in the analysis of multi-rotor UAVs is first introduced,and the flight principle of the tilt tri-rotor UAV is emphatically analyzed.Based on the Newton-Euler equation,the dynamics and kinematics of the quadrotor UAV and the tri-rotor UAV are built based on Euler angle representation and unit quaternion representation respectively.Then the fault dynamics of the multi-rotor UAV under the actuator’s fault are deduced for convenience of the following controller design.2)To solve the attitude control of the quadrotor UAV under the circumstances of the actuator’s partial loss of effectiveness,the fault dynamics is obtained by adding the actuator’s fault to the system model as a multiplicative factor.Then a PFTC strategy based on adaptive sliding mode algorithm is designed considering the unknown external disturbances in the same time and the asymptotic stability of the designed controller is proved using Lyapunov-based method.Finally,the control strategy is verified on the quadrotor hardware-in-the-loop-simulation(HILS)testbed,and the results show that the proposed method can effectively counteract the external disturbances and actuator’s fault.3)Considering the same problem as the above,the fault dynamics are firstly improved through multiplying the fault matrix directly with the thrust vector.Immersion and invariance(I&I)based observer is introduced to estimate the actuator’s fault in real time,and then a state estimation vector is introduced to deal with the problem that the partial differential equation is sometimes unsolvable.To compensate for the mismatch between the state vector and the state estimation vector,a dynamic factor is designed to ensure the estimation errors asymptotically converge to 0.Based on the analysis of the parameters’ boundedness,sliding mode based fault tolerant controller is utilized to compensate for the actuator’s fault,and the system stability is proved through Lyapunovbased theory.Real time experiments are implemented on the HILS testbed,and the better performance is validated comparing with the traditional sliding mode control algorithm.4)Considering the attitude control of the tri-rotor UAV after the rear servo’s stuck fault,an adaptive sliding mode observer is first designed to estimate the unknown rear servo’s stuck fault,and then robust integral of the signum of the error(RISE)based fault tolerant controller is designed to compensate for both the estimation errors and external disturbances.The stability of the closed-loop system is verified via Lyapunov-based analysis.To improve the fidelity and efficiency of the flight control test of the tri-rotor UAV,a HILS testbed of the tilt tri-rotor UAV is designed and developed.The HILS real time experiment verifies the effectiveness of the proposed method and the comparison results with classical PID control algorithm and feedback linearization method show that the proposed FTC strategy has better control performance against the rear servo’s stuck fault.5)Aiming at the attitude and position control of the tri-rotor UAV after the rear servo’s stuck fault occurs which is a typical underactuated problem,the dynamic model of the tri-rotor UAV is divided into the altitude subsystem,the outer-loop translational position subsystem and the inner-loop attitude subsystem.The altitude subsystem and the outer-loop translational position subsystem utilize I&I based adaptive control algorithm to compensate for the uncertainty of the model parameters,so that the outer-loop virtual control input and the inner-loop control object is obtained.The inner-loop firstly utilizes an adaptive sliding mode observer to estimate the unknown rear servo’s stuck fault,and then the RISE control algorithm is adopted to compensate the estimation errors and unknown external disturbances.The stability of the double loop is verified using Lyapunov-based analysis.The HILS real time experiment shows that the proposed FTC strategy can properly deal with the rear servo’s stuck fault. |
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