FEM Simulation and Reverse Engineering of an Electronic Railway Wheel Detector
The electronic wheel detector is a device that has been widely used at SNCF Company for train passage detection, which constitutes a primary task in coordination of trains' signalization. The detectors have been deployed in the company for at least three decades; however, some trouble has been remarked in their functioning apparently due to their sensibility to external magnetic fields. This is why section in EMC studies at SNCF Company was requested to start developing a model of such detectors, so simulation scenarios reproducing their functioning could be implemented to determine potential sources of malfunctioning. Nevertheless, little information concerning detectors themselves is available: due to their oldness, they're no longer fabricated and only few technical documents describing their functioning remain. In consequence, the data we comprised of for developing this model were a brief description of detection principle as well as a physical exemplary which dimensions we could use for building our model's geometry.
These detectors mainly consist in a "U" ferrite with two coils connected in series in a way the magnetic flux produced by their current is additive. They're as well connected in series to a capacitor, so they form an LC oscillating circuit which resonating frequency is typically 39 kHz or 50 kHz. When a train passes, the "U" is closed by a magnetic material, which changes the overall magnetic flux and thus, the equivalent inductance of the circuit. This results in a change of the circuit's resonating frequency that is further translated into a train's passage.
Such device can be modeled by using the COMSOL Multiphysics® AC/DC module. In this module, the physics that best adapt to our modeling problem are implemented in the Magnetic Fields and Electrical Circuits interfaces. These interfaces allow calculating the magnetic field generated by the detector as well as implementing an RLC oscillating circuit which "L" parameter is determined by the geometric coils defined in the 3D model and which "C" parameter is known since it is indicated in the physical device we dispose of and can be modeled as a lumped element.
A first transient simulation was performed to determine how appropriate the detector's model is with respect to its theoretical functioning. For this purpose, the voltage excitation signal in the circuit was defined as a unit echelon, and since the system operates at a linear point for the considered excitation, a linear response is expected. Such response would consist in a transient oscillatory signal which pseudo-frequency should correspond to the resonating frequency of our detector (which is 39 kHz).
First performed simulations show that our model corresponds to the detector's theoretical characteristics. However, little experimental data allowing our model's validation is available; this is why further work includes cooperating with SNCF's measurements and experimentation department. Once the model validated, we'll be able to use it in more complex simulation scenarios leading to a better understanding of the detector's malfunctioning sources.
