Semiactive Control of Hunting Stability in Rail Vehicles

This paper discusses a methodology for improving hunting behavior in rail vehicles by semiactive control of suspension elements. This methodology focuses on increasing the critical velocity of hunting beyond the operational speed range. Earlier studies have shown that the critical velocity is most sensitive to the primary longitudinal stiffness, KPX. A higher primary longitudinal stiffness can significantly increase the critical hunting velocity of the rail vehicle. But, having high values of KPX is largely undesirable, since that would make the wheelset suspension very rigid. Any disturbance to the wheels due to imperfections in the rails will result in a forcing function in the equations of motion, thereby facilitating the transfer of the recurrent wheelset oscillations to the car body, leading to a poorer ride quality. As an alternative to using sustained high values of KPX throughout the ride, a semiactive approach has been attempted, whereby, a nominal value of KPX was assumed for the majority of the simulation time. On a need basis, this value was made to increase for limited portions of the oscillatory cycle. As an initial approach, the value of KPX was made to be a function of the wheelset lateral excursion. A moderate value of KPX was assumed for the duration of time that the wheelset lateral excursions were below a preset threshold value. Whenever the lateral displacement exceeded this threshold limit, the value of KPX was increased to 10 times the original value. It was thought that this approach would raise the critical hunting velocity, and drive the oscillations down towards the center. But simulation showed that this strategy barely improved the critical velocity. A subsequent approach was to make the value of KPX a function of the wheelset yaw displacement instead. This approach yielded a significant improvement in the critical velocity.