Creating a Test Bench for Railway Wheel and Track Contact Research Using CompactRIO and LabVIEW

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"With NI products, we have the flexibility to meet our increasing scope of instrumentation needs using the work we have already completed. We could use LabVIEW to swap platforms if necessary, migrating to PXI equipment, for example, at a very low development cost."

- Eduardo Gómez, CEIT - Centro de Estudios e Investigaciones Técnicas de Guipúzcoa

The Challenge:
Developing a test bench to analyze the contact conditions of railway carriages and thoroughly study the contact between the wheel and the track, which is key to solving railway problems as well as developing new traction and brake control algorithms for different kinds of carriages.

The Solution:
Using NI CompactRIO hardware and NI LabVIEW software to build a flexible test bench that we can customize as needed.

Author(s):
Eduardo Gómez - CEIT - Centro de Estudios e Investigaciones Técnicas de Guipúzcoa
Asier Alonso - CEIT - Centro de Estudios e Investigaciones Técnicas de Guipúzcoa

The CEIT Applied Mechanics department has more than 25 years of experience developing R&D projects in the railway sector. One of our areas of expertise is in the phenomena associated with the contact between the wheel and track. The wheel/track contact is the most differentiating aspect of the railway. Its exact value is critical when fabricating vehicles to meet the requirements expected in the design stage.

We created a test bench for the experimental study of the wheel/track contact based on our theoretical knowledge. The main objective of the test bench was to obtain the curves that link pseudosliding (longitudinal, lateral, and spin) in the contact with the forces (vertical, lateral, and longitudinal) originated for a predetermined forward speed. From a mechanical and instrumental point of view, we needed to develop a specific test bench to reproduce and monitor these conditions. Currently, there are no solutions on the market to configure these conditions.

Test Bench Description

To reproduce the described test conditions at a low cost, we opted for a solution where a roller substituted as the track and we controlled the vertical load applied to the contact as well as the turning speed. Therefore, the test bench had to accurately measure and control these two parameters. Additionally, we had to maintain the vertical load as stable throughout the test.

The system indirectly controlled the other parameters as follows:

  • Longitudinal pseudosliding depends directly on the turning speed difference of the two rollers. We controlled this difference by maintaining the turning speed at a constant level in one of the rollers while we applied controlled braking to the other roller. It also required the exact measurement of the speed difference between both rollers.
  • Lateral pseudosliding is the attack angle between both rollers, equivalent to one situation on the via curve in the railword, which causes the pseudosliding to vary. Therefore, the test bench had to continuously adjust the value of this attack angle.
  • Spin is directly related to the angle formed by the norms of both surfaces on the contact point. The only way to change this is to change the geometrical shape of at least one of the rollers. 

We designed the test bench shown in Figure 1 based on these needs. The test bench comprised two 69 kW engines. One of the engines acted as the tractor engine and the other as the break system, with both engines in high-dynamic capacity. To reduce power use, there was regeneration between both engines. One of them was linked to the roller that emulated the wheel and the other to the roller that emulated the track.

The vertical contact load was applied with a pneumatic balloon that acted as suspension and maintained load stability during the test. The system measured the three components of the real contact force with an error margin of less than 2 percent. Both rollers were easily replaceable, and we could adapt the geometrical form of each one according to the test performed. Also, we could change the attack angle for one of the rollers with regard to the other one.

Measurement and Control System Architecture

From a mechanical point of view, the test bench featured a large variety of control parameters. To ensure that the test bench had the same flexibility when it came to signal acquisition and control, we chose CompactRIO and LabVIEW for the following fundamental reasons:

  • CompactRIO and LabVIEW reduced the acquisition and control software development time to a minimum. We had less than three months to implement all the software, including the operating tests.
  • With NI products, we have the flexibility to meet our increasing scope of instrumentation needs using the work we have already completed. We could use LabVIEW to swap platforms if necessary, migrating to PXI equipment, for example, at a very low development cost.

Figure 2 shows the sensors and actuators implemented on the test bench, as well as the general architecture in a simplified form. PROFIBUS carried out the communication between both engines on the test bench. With this protocol, we could obtain valuable additional information about the engines. The attack angle was controlled via PWM through an amplifier, and was measured at high precision with an encoder also connected to the PROFIBUS network.

The system measured the roller speed with two types of independent sensors to obtain the highest possible precision. All control loops and communications in CompactRIO were implemented in real time at 1 kS/s. The remote control post was based on an external PC that performed noncritical operations.

Specifically, we used the NI cRIO-9073 integrated system connected to an extensometer, analog and digital modules, and PROFIBUS (master/slave) modules, with slots available to increase the capacity of the system. We met the required control and acquisition specifications through the possibility of implementing code for the FPGA and for the CompactRIO real-time processor.

Fast Development Time, Flexible System

Because of the complexity of the research project, we required a flexible test bench. We were limited by development time and equipment costs, mainly because the research work was more about comparing the theoretical and experimental results than about the test bench itself. In this sense, the combination of hardware and software offered by National Instruments facilitates this type of development. We spent a larger amount of time on the research itself instead of on the implementation. We finished the implementation work in two months, one month ahead of schedule, and took the test bench to the InnoTrans 2012 international railway convention in Berlin.

Author Information:
Eduardo Gómez
CEIT - Centro de Estudios e Investigaciones Técnicas de Guipúzcoa
Spain

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