Study On Complex Behavior Characteristics And Diagnosis Methods Of Hunting Stability For High-speed Emus

With the rapid increase in running speed and operating mileage as well as the continuous change in line condition,the lateral stability of high-speed EMUs is facing new challenges,and a series of problems including bogie alarm and carbody swaying caused by hunting instability have appeared successively.Hunting stability,one of the most important factors affecting the vehicle's running safety and ride comfort,determines the critical running speed of railway vehicles.The bogie hunting instability often appears in the later period of wheel reprofiling.To avoid the bogie hunting instability of high-speed EMUs,the wheel reprofiling cycle of EMUs is shortened substantially,which increases its operational cost significantly.The carbody hunting instability often appears in the early period of wheel reprofiling and disappears gradually with the increase of operating mileage.The carbody swaying caused by carbody hunting instability still occurs occasionally,and becomes the main factor to deteriorate the ride comfort.Therefore,further research on the hunting stability of high-speed EMUs is carried out to provide the optimization measures for improving both bogie and carbody hunting stabilities by means of exploring the complex behavior characteristics and the internal mechanism of the bogie and carbody hunting instabilities,which has important theoretical meaning and practical value to the running safety and stability of high-speed EMUs.Furthermore,it is urgent for the sustainable development of high-speed EMUs to establish the monitoring and evaluation method of hunting instabilities starting from its behavior characteristics and to achieve the transition from a preventive maintenance strategy to a predictive maintenance strategy.In this thesis,focusing on the hunting stability issue of high-speed EMUs,the following research work is carried out:(1)The approximate analytical expressions for linear critical speed and hunting frequency of the wheelset and bogie systems with two degrees of freedom are derived,and the influences of relevant parameters including the gyro moment on the critical speed and hunting frequency are analyzed.Considering the stiffness characteristic of the yaw damper,the approximate analytical expression for hunting frequency of the high-speed vehicle is derived,and the influences of suspension parameters on the hunting frequency are studied.The results show that the gyro moment plays a positive role in stability and its stabilizing effect strengthens with the increasing running speed.The hunting frequency is not only related to the speed and equivalent conicity,but also closely related to the suspension parameters,especially the longitudinal stiffness of the axle box and the parameters of the yaw damper.(2)The lateral dynamic model for the linear system of the high-speed EMU is established.The behavior characteristics of modal parameters before and after bogie hunting instability and carbody hunting instability are studied using the root locus method and the modal tracking method.Considering the large fluctuations of the equivalent conicity and the running speed,two objective functions related to the minimum damping ratio are proposed to represent the degree of bogie hunting stability and carbody hunting stability respectively.Starting from objective functions,the influence mechanism of suspension parameters on the bogie and carbody hunting stability is studied,and the suspension parameters are optimized by genetic algorithm.The results show that when the carbody hunting instability of the high-speed vehicle occurs,the frequency and vibration mode conversions between the bogie hunting mode and carbody natural mode will appear,and the damping ratio of the bogie hunting mode decreases while that of the carbody natural mode increases.The influence mechanism of equivalent conicity on the bogie and carbody hunting stability differs in presence of different values of suspension parameters.Moreover,both the bogie hunting stability and carbody hunting stability can be improved synchronously by optimizing the suspension parameters.(3)Considering the flexibility of rails and vehicle components,the nonlinear dynamic model of the high-speed EMU is developed on the basis of the measured wheel-rail profiles and the characteristic curves of suspension components.Both the bogie and carbody hunting instabilities are reproduced within the range of normal operating speeds,and the complex dynamic behaviors before and after the hunting instability are studied,including the bifurcation characteristic of the wheelset,the vibration spectrum characteristics of the bogie frame and carbody,ride comfort,running stability,running safety,wheel-rail contact relationship as well as the vibration mode of carbody.The results show that under the carbody hunting instability condition,both the wheelset and carbody possess a large amplitude movement in the lateral direction,and the wheel flange contact may occur,which leads to the instantaneous value of the derailment coefficient exceeding 0.8.Both bogie and carbody hunting instabilities will deteriorate the lateral ride comfort.The phenomenon of frequency capture appears in the cases of carbody hunting instability.During the carbody hunting instability,the bogie hunting frequency is captured by the carbody natural frequency.(4)Based on the synchronous vibration behavior during hunting instabilities,the monitoring method for hunting stability of high-speed EMUs is studied from the viewpoint of signal correlation analysis.Considering the vehicle operating conditions of stability,small amplitude hunting instability and large amplitude hunting instability,the correlations between vibration signals in different positions and different directions are analyzed,and the most effective cross-correlation indicator and its critical value in monitoring hunting stability is determined on the basis of the obtained results and the comparisons.The proposed detection method is validated by means of field test data.The results show that the correlation coefficient between the lateral accelerations of the bogie frame end and the carbody floor can be used to monitor the bogie hunting stability and carbody hunting stability simultaneously.The critical value of cross-correlation coefficient for the bogie hunting instability is 0.5 while that for the carbody hunting instability is 0.8.(5)Starting from the characteristics of bogie hunting instability and the running safety of the vehicle,a new evaluation method for bogie hunting instability of high-speed EMUs is proposed on the basis of Hankel-SVD.In this method,the wheelset movements can be predicted by Hankel matrix and singular value decomposition method by using the lateral acceleration of bogie frame end as input,and the real-time analysis of wheel-rail contact relationship is realized by lookup table method.According to the relationship between the wheel-rail contact position and the derailment coefficient,the correlation between the acceleration of bogie frame end and the running safety of the vehicle is developed.Finally,the evaluation method of bogie hunting instability is proposed based on this correlation.The simulation and experimental results show that the proposed method can predict the wheelset movement and wheel-rail contact relationship during the bogie hunting instability successfully.(6)A peak count based carbody hunting instability evaluation method for high-speed EMUs is proposed by considering its characteristics and combining with the frequency weighting function of the existing ride comfort evaluation methods.This method establishes the frequency-amplitude function of the carbody hunting instability by the classification standard of the lateral ride comfort index and the analog signal.The classification evaluation of the carbody hunting instability is achieved by statistical analysis of the peak value of the lateral acceleration on the carbody floor.