Road test data procedures for evaluating the Hunting instability thresholdof a railway vehicle from on board measurements

In this paper, an original method is proposed which allows to estimate the modal parameters of a railway vehicle running on a real railroad line. Base d on on-line measured acceleration, the methodology, hypothesising the rail way corrugation as a random process, can separates the effects of the force d and the free response of the vehicle. Analysing the axle-box measured acc eleration, the Random Decrement Technique provides the free response in ter ms of autocorrelation function. From the obtained autocorrelation function, with time domain modal parameters identification methods, the natural freq uencies and the related damping factors can be estimated by means of Ibrahi m and Prony methods without any numerical modelling. In the paper the compa rison between stability analysis using the identified damping factors and t he traditional acceleration RMS value, both in stable and unstable conditio ns are reported. Experimental results, related to different railway vehicle configurations, are analysed.