Research On Robust Flight Control Of Small-Scale Unmanned Helicopter In The Presence Of Mismatched Disturbances

The small-scale unmanned helicopters have the unique capability of vertical taking-off and landing,hovering,high levels of agility and maneuverability.Therefore,they have broad application prospects in military and civil fields.However,the small-scale unmanned helicopter inevitably suffers from model uncertainties and external wind gusts in flight.More particularly,these lumped disturbances can be classified into matched disturbances and mismatched disturbances.Therefore,it has become one of the research focuses in the academic circle to design high-performance flight control system for small-scale unmanned helicopter.As to the above characteristics of the small-scale unmanned helicopter,the main works of this dissertation are research on robust flight control algorithm,and it proposes several high-performance control algorithms.Specifically,the major contributions of this work can be summarized as follows.Firstly,based on the previous achievements on the dynamic modeling of small-scale unmanned helicopter,this dissertation introduces the nonlinear dynamic model of small-scale unmanned helicopter in detail.By synthesizing the rigid-body kinematics,rigid-body dynamics and main rotor flapping dynamics,we establish the complete nonlinear dynamic model of the small-scale unmanned helicopter.Furthermore,with the in-depth analysis on the force and moment generation processes,the preliminary simplified model is developed,which lays the foundation for the flight controller design in the following sections.Secondly,a two-layer flight controller is designed for the small-scale unmanned helicopter based on the linearized model.Since the inner-loop system suffers from mismatched disturbances,we reconstruct an equivalent system,which divides the mismatched disturbances into the major matched component and minor mismatched component.The matched component is compensated by feedforward control method,and the compensating error and minor mismatched component are suppressed by H_∞ control algorithm.Meanwhile,the outer-loop controller is designed with the dynamic inversion technique.Simulation results demonstrate the superior control performance and strong robustness to model uncertainties and external disturbances of the proposed flight control scheme.Thirdly,a novel sliding mode control(SMC)method is developed for the small-scale unmanned helicopter in the presence of both matched and mismatched disturbances.The feedback linearization technique is proposed to linearize the approximate nonlinear model.A new global sliding surface is deigned by incorporating the estimation of the mismatched disturbances and their derivatives,and a novel sliding mode control law is developed.Theoretical analysis and numerical simulation results demonstrate that compared with traditional SMC method,the proposed novel SMC method can achieve better tracking performance.Finally,the multivariable super-twisting algorithm is known as a second-order SMC algorithm.Therefore,it can suppress even eliminate the inherent chattering phenomenon in first-order SMC algorithm.Some modifications are made for the existing multivariable super-twisting algorithm to restrain the disturbances of the unmanned helicopter more efficiently.The backstepping method is proposed to build up the structure of the controller,and to handle the under-actuated problem.With the estimation of the disturbances derived by modified multivariable super-twisting observer,the disturbances are compensated by feedforward method.And the compensating errors also are suppressed by the modified multivariable super-twisting control algorithm.The global asymptotic stability of the closed-loop system is proved via the Lyapunov theory.Furthermore,simulation results demonstrate the superior control performance and strong robustness to both matched disturbances and mismatched disturbances of the proposed flight control strategy.