Online assessment of train hunting stability by monitoring dynamic wheel–rail displacement: why and how?

Assessing the hunting stability of trains in real-time is required to ensure their safe operation. In existing standards, the methods for determining the hunting instability of rail vehicles are mainly based on evaluating the acceleration of bogie frames or carbodies. In this paper, the limitations of such acceleration-based methods are firstly highlighted by multibody dynamics simulations of a high-speed vehicle, and a novel signal source, dynamic wheel–rail displacement (DWRD), applicable to assessing the hunting stability is proposed. Then, based on a simplified YOLO model and an improved UNet model, a virtual point tracking (VPT) method is proposed for processing the wheel–rail contact video to obtain the DWRD. Finally, the VPT method is used to monitor the DWRD of a vehicle subjected to a destabilization experiment on a full-scale roller rig, and the resulting DWRD is further used to assess the hunting stability of the vehicle, while the acceleration-based methods are presented for comparison. The results show that the vehicle experiences a noticeable carbody hunting motion with a frequency of approximately 1.1 Hz, and the carbody acceleration, although reflecting this dominant frequency, has an amplitude lower than the limit value of 0.5 \({\text{m}}/{{\text{s}}}^{2}\) $\text{m}/{\text{s}}^2$ ; the bogie frame acceleration, in addition to containing this dominant frequency, also has other noticeable dominant frequencies, e.g., 9.4 Hz, and additionally has an amplitude noticeably lower than the limit value of 8 \({\text{m}}/{{\text{s}}}^{2}\) $\text{m}/{\text{s}}^2$ . Compared with the two acceleration-based methods, the DWRD-based method not only accurately reflects the hunting frequency of 1.1 Hz, but also reflects the wheel–rail gap, thus directly and reliably reflecting the wheel–rail contact state. Further, a criterion for judging the hunting stability is proposed from the perspective of DWRD, i.e., the vehicle is judged to be unstable once the DWRD exceeds the threshold of \({Y}_{0}/\alpha \) $Y_0/\alpha$ for six consecutive times, with \({Y}_{0}\) $Y_0$ and \(\alpha \) $\alpha$ being the nominal wheel–rail gap and a safety factor, respectively.