Design Analysis And Motion Control Techonology Research Of Tri-Tilt Rotor UAVs

Since the fixed-wing unmanned aerial vehicles(UAVs)are incapable of vertical take-off and landing(VTOL)while the multi-rotor UAVs have the flaws of low speed and inefficient flight,the tilt rotor UAVs have attracted considerable attention in recent years.The tri-tilt rotor(TTR)UAVs not only have the merits of VTOL and easy manipulability that of the rotor aircrafts,but also have the merits of long endurance and fast speed that of the fixed-wing aircrafts.Compared with the twin and quad-tilt rotor UAVs,the TTR types have superiorities in energy utilization and flight stability.Although the TTR UAV extends the application range of the UAV,its stable flight and altitude maintenance control in transition mode(TM)are always challenging problems.In the TM,the UAV is affected by not only the internal disturbances(unmodeled dynamics,nonlinear couplings and model parameter perturbation,et al)and the external disturbances,but also the disturbance torques from tilting rotors,control redundancy(CR)and lift transformation(LT)that only exist in the TM.Thus,this dissertation designs a TTR UAV with good manipulability.Targeting the transition procedure from rotor mode to fixed-wing mode,a disturbance compensation nonlinear control scheme is proposed such that stable flight and altitude maintenance can be realized.Main works and contributions are summarized as follow:(1)Aerodynamic interferences between the tilting rotors and the wings are analyzed by using CFD methods,such that a configuration scheme of the TTR UAV is obtained.Then influences on the rotor wake from the wings are degraded in hover flight,which in turn improves the aerodynamic efficiency of the rotors.(2)Mechanisms of ascending/descending,roll,pitch and yaw movements of the TTR UAV are analyzed.Six degree-of-freedom(DOF)model of spatial movement of the TTR UAV is established.Since the mode transition is accomplished in longitudinal symmetrical plane of the UAV,the six DOF model is simplified into a three DOF one by neglecting the lateral movement,which gives the basis of designing flight control system of the TTR UAV in the TM.(3)Targeting the TTR UAV pitch angle system that belongs to a class of generalized Lipschitz conditions satisfied-affine nonlinear system,a novel disturbance estimator(DE)is proposed to estimate the internal and external disturbances.Based on the analysis of the disturbance feedback compensation theory,the disturbance feedback compensation scheme is designed such that robustness of the pitch angle system against disturbances is enhanced.Drawbacks of the commonly used disturbance observers(DOs),for example,the multiple-input-multiple-output(MIMO)systems are not applicable and the estimation capability of non-smooth disturbances are very poor,are also addressed,such that the application range is extended and the estimation accuracy is significantly improved compared with the commonly used DOs.(4)Targeting the sudden change of the disturbances encountered by the pitch angle system of the TTR UAV,two predictive controllers that overcome elevator surging/saturation in a very short time are proposed.Limitation of control methods of conventional predictive approaches are also overcame.(5)Targeting the difficulty obtaining of the accurate position model problem,the planar guidance algorithm that can deal with planar curved path following problems is applied to the altitude control of the tilt rotor UAVs for the first time.The connection between position system and attitude system is built without using any model information of the position system.The design procedure of the altitude maintenance control system is also simplyfied.Common flaws of similar approaches such as poor adaptation,low reliability,to many adjustable parameters with more complicated tuning processes are addressed.(6)Targeting the MR and LT problems in the TM,effects from different control surfaces on the altitude of the UAV are analyzed.The essential reason of the altitude alteration is also obtained.By introducing the control of forward velocity,an altitude maintenance fully autonomous control scheme suitable for different kinds of tilt rotor UAVs is proposed.The problems of difficulty determination of weight coefficients in semi-autonomous assignment of total control input and system instability in adaptive assignment of total control input are all addrerssed.