THE IDEA: ROBUST SENSORS FOR THE NEXT GENERATION OF SIGNALING SYSTEMS
Figure 1 A modern rail line, with a Frauscher wheel sensor (yellow box) in situ.
The safety and integrity of the modern rail network is possible thanks to an extensive system of sensors that are constantly reporting the location, speed, direction and state of trains. Frauscher Sensor Technology has developed and manufactured high-quality products in the field of inductive sensor technology since 1987, with a focus on railway signaling and train control systems that are responsible for the safety of rail operations.
Wheel sensors are key components of these systems. These identify and count the wheels that pass over them, allowing signalers to see which sections of tracks are occupied and making it possible for equipment such as level crossings to be activated automatically. Frauscher’s wheel sensors are based on induction, and work by measuring the changes in complex impedance of inductive coils as the metal train wheel passes over....
THE CHALLENGE: RELIABLE SENSING OF TRAIN WHEELS IN CHALLENGING CONDITIONS
Most rail infrastructure is exposed to the elements, which means that it is exposed to extreme variations in temperature and humidity while rain, snow, ice and sand can accumulate around the equipment.
The trains themselves also introduce additional challenges, including misalignment due to cornering and spurious signals from eddy current brakes. Frauscher is a participant in the FP7 European ECUC (Eddy-Current Brake Compatibility) project. The objective of this project is to prove that linear eddy-current brakes (ECB) are an effective and applicable solution for increasing the braking capacity of new high speed trains and solving the concerns raised by railway infrastructure managers to overcome the drawbacks that ECB have experienced on some lines.
Because rail signaling equipment must operate to safety integrity level 4, the highest level, anything that might impair sensor performance needs to be carefully analyzed. For this reason, Frauscher introduced electromagnetic simulation with CST Studio Suite® into their development process.
By constructing a virtual prototype in CST Studio Suite (Figure 2), Frauscher were able to model the sensor response in a variety of conditions. The LF frequency domain solver was used, due to its ability to calculate eddy currents and AC coils. Automatic parameter sweeps made it possible to calculate how the sensor response varies as the wheel passes over the sensor and to calculate how the alignment of the wheel flange might affect performance on corners. Each individual simulation took just 10 minutes, allowing complex studies to be performed in a reasonable timescale.
Figure 2 A simulation model of the RSR123 wheel sensor, with a section of rail and a wheel. One coil is shown.
Figure 3 Magnetic field lines, showing coupling through the wheel.
THE RESULT: SAFE, VERSATILE WHEEL SENSORS SUITABLE FOR THE MODERN RAIL NETWORK
The agreement between simulation and measurement (Figure 4 and Figure 5) gave Frauscher confidence in the ability of the LF frequency domain solver in CST Studio Suite to model its sensors. This paved the way for more complex analyses, such as characterization of the parasitic capacitance of the coils, and investigation of how the effect of age and moisture on materials might influence the performance of the sensors. Frauscher was also able to export simulation results into SPICE and MATLAB for further analysis.
Figure 4 Response curves for one system in an RSR123 wheel sensor with three different wheel alignments, comparing measurements (solid lines) with simulation (points).
Figure 5 Response curves for both systems in an RSR180 wheel sensor, comparing simulation (solid lines) with measurements (points).
With the help of simulation, Frauscher was able to better understand the physical working principles of the sensors and the interaction between sensor, rail and wheel. This helped it to ensure that its wheel sensors met the stringent safety standards of the rail industry, and to begin work on its next generation of sensor devices. Simulation reduced prototyping time and costs for future development, and allowed engineers to consider mounting configurations that otherwise would be prohibitively expensive or physically not possible.