Inverse Identification of Dynamic Wheel-rail Contact Forces

Accurate evaluation of contact forces between wheel and rail is essential in the assessmentof vehicle performance and to predict consequences of dynamic vehicle-track interaction.As the contact forces can not be measured directly in the field, one common approachis to measure the strain or acceleration at various positions on a wheel or wheel axle.Based on this data, the forces can be estimated. However, the existing schemes typicallyinvolve either a simplified wheel model (neglecting inertia) or, in the case of using moreadvanced models, imply strong restrictions in terms of the choice of spatial and temporaldiscretization of the underlying equations of motion.In this work, the vertical contact force is determined by the solution of an inverse problem.A minimization problem is considered in which the time-history of the contact forceis sought such that the discrepancy between the predicted and the measured response(strains) is minimized. A particular feature of this formulation is that the discretization ofthe pertinent state equations in space-time, the sampling instances of the measurementsand the parameterization of the sought contact force are all independent of each other.Additionally, the convergence of the spatial and temporal discretization of the model andthe time parameterization of the contact force history are investigated.The proposed strategy is firstly evaluated for a simplified 2D disc with focus on the effectsof discretization, sensitivity to noise and possible improvements as a result of properregularization. Effects of considering different measurement outputs for the minimizationproblem are investigated. In particular, the identification strategy is modified by applyingvirtual calibration whereby an apparent static load is used as the measurement output,in order to compensate for model and spatial mesh sensitivity.Considering the realistic problem at hand, the rotating wheel is introduced and themeasured strains are combined using two Wheatstone bridges to estimate the contactforce by the static calibration technique. The inverse identification strategy is adoptedfor the designed measurement system. Effects of centrifugal and gyroscopic terms inthe equation of motion and consequences of noise in the measurement data are evaluated.Finally, the inverse identification strategy is compared to static calibration and a Kalmanfiltering technique for realistic load cases