Research On Unmanned Aerial Vehicle Low-Frequency Braking And Ground Taxiing Stability

UAV(Unmanned Aerial Vehicle)take-off and landing process is one of the most important phases during the whole flight.The frequent accidents including runway excursion,rollover and landing gear structure fracture during the ground rollout process significantly affect the UAV safety.The development of the upper air high-speed UAVs and the spring-up of the new control systems and materials technology raise new challenges to the performance of the aircraft landing gear and the ground control systems.Therefore,investigation of the dynamics,stability and control problems resulting from the UAV brake and ground rollout process is quite significant not only in the landing gear and the ground control systems design,but also in the improvement of the UAV ground taxiing stability.The stability of the slip-ratio-control,the acceleration-control and the combined electric braking systems is analyzed based on the Routh Criterion and the Lyapunov stability analysis methods.The restrictions ensuring the braking system stability are obtained theoretically.Considering the velocity sensor noise,a combined anti-skid braking control law is designed and the three-DOF(Degree Of Freedom)UAV ground rollout model is simulated under different runway surfaces.The results show that the designed braking control system is able to improve the system stability and to reduce the braking oscillation.It is proposed to adopt the bifurcation theory to conduct the research on the UAV nonlinear ground steering stability problems.The six-DOF UAV ground rollout model is established and the numerical continuation method is used to analyze the UAV steering motion along with the system parameters’ change.The influences of the uniform rectilinear velocity,the steering velocity,the nose wheel steering angle,the rudder area,the distance between the two main wheels and the tire friction coefficients on the UAV steering stability are studied.The bifurcation diagrams are obtained and then,the dynamics and kinetic characteristics are investigated.It is revealed that the essence of the steering instability is that the saturation of the tire lateral force can not provide with enough centripetal force for the UAV circular motion.In order to increase the accuracy of the UAV whole model and to reduce the risk of the UAV full scale tests,the multi-disciplinary collaborative simulation technique is introduced to study the comprehensive direction control system performance.The rigid-flexible coupling high-speed UAV rollout virtual prototype is built in LMS Virtual.Lab Motion.And the model analysis of the landing gear is carried out in Hyper Mesh/NASTRAN.Then the electric braking system,the aerodynamic force and the control surfaces modules are all established in MATLAB/Simulink.Also,the anti-skid braking control law and the designed comprehensive direction control law are written by C-language in Visual Studio.All the four parts above form the collaborative simulation model and the simulation is conducted through the data interfaces among the softwares.In addition,the optimizing method is used to design the active comprehensive direction control system after studying the performance of the rudder system,the nose wheel steering system and the braking system.Finally,the comprehensive direction control system is verified to be stable and efficient under different working conditions with various lateral disturbance.The virtual prototype technique and the Lagrange’s second equation are both adopted to build the landing gear low-frequency brake-induced vibration analysis model.Then the mathematical model is verified to be valid,which can be used to study the gear walk performance.The coupling interaction between the gear walk dynamic response and the braking control system is pointed out.The influence of the landing gear longitudinal motion on the main wheel slip ratio is added in the gear walk model.Then the gear walk vibration mechanism is analyzed and the effects of the landing gear structure parameters and the braking torque are studied.The short-time Fourier transform method is used to capture the gear walk magnitude-frequency characteristic in the whole time domain.The performance criteria are designed and then the parameter sensitivity is analyzed.Based on the multidisciplinary optimization method,a braking control law combining the feedforward and the feedback control is proposed with significant improvement of the gear walk performance.Then the Pareto optimal solution set is obtained via multi-objective optimization.This allows the trade-off between the braking efficiency and vibration suppression performance to be demonstrated.Finally,two traditional anti-skid braking control systems and a new combined braking control system based on the intelligent control theory are applied to the gear walk model.The effects on the gear walk performance are compared and the combined braking control law is verified to be of great stability and adaptability under variable external conditions.