Upgrade of a Rail and Sleeper Fatigue Test System Using NI PXI, SCXI, and LabVIEW

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"PXI, SCXI, and LabVIEW were the ideal platform for upgrading our client’s decades-old test rig. A key benefit was the shift from a traditional measurement system, which occupied two big panels, to a single tabletop system. We also moved from bulky control panels, plotters, and oscilloscopes to a single intuitive GUI displaying all of the computer-based control facilities and options."

- Sriram Iyer, Apna Technologies and Solutions Pvt Ltd

The Challenge:
Refurbishing the measurement and control system of a rail and sleep fatigue test rig with state-of-the-art technology and an intuitive GUI to control four independent hydraulic actuators at a user-defined frequency, amplitude, and waveform based on the displacement of the piston or load applied on the unit under test.

The Solution:
Using NI PXI and SCXI with the LabVIEW Real-Time Module and the LabVIEW PID and Fuzzy Logic Toolkit to independently control the four actuators in synchronization.

Author(s):
Sriram Iyer - Apna Technologies and Solutions Pvt Ltd
Starlin Immanuel - Apna Technologies and Solutions Pvt Ltd
Senthil R. Desappan - Apna Technologies and Solutions Pvt Ltd

Apna Technologies and Solutions Pvt. Ltd. provides knowledge-engineering products and custom railway solutions. We developed a project under contract from IIT-Kanpur, with the guidance of Dr. Sanjay Gupta. Our client, Indian Railways, had a 1980s test rig set up to test the fatigue life of railway tracks and sleepers under various cyclic loading patterns by studying the crack initiation and propagation under vertical and lateral cyclic loading.

To keep up with the growing demand for precise control and faster turnaround time, the client wanted to replace the traditional system, consisting of several racks of control units and oscilloscopes, with an integrated DAQ and control unit and a PC-based user interface.

System Description

The system controls four independent hydraulic actuators through ±10 V analog output signals. The cyclic loading-pattern control is based on feedback from load cells through an NI SCXI-1520 module for load control, or from the linear variable differential transformers through an NI SCXI-1540 module for displacement control. The user can define the cyclic waveform (such as triangular, sinusoidal, or square), amplitude, and frequency of the desired loading pattern. The system also can superimpose the cyclic loading over a static load.

The system communicates with several peripheral programmable logic controllers (PLCs) used for controlling pumps and motors. The GUI is a menu-driven application with all functions available from the main screen (Figure 1). The user can configure each channel (input and output) and input calibration through the Settings and Config. Acq. buttons.

The user can also set system safety limits for each parameter to avoid accidents during tests (see Figure 2). The LabVIEW Real-Time Module checks these settings at all times when the machine is on. If any parameter exceeds the safety limit, the system automatically launches the safe-turn-off sequence.

The real-time controller we use adds value to the control and safety features. The features run in a timed While Loop at 2 ms. For the application, it was imperative that the loop run at exactly 2 ms because actuator movement and system results depend on output. This level of reliability was possible only with a real-time controller and OS. The real-time controller also monitors the sensor inputs and safety and emergency signals every 2 ms to make the operation of the rig as safe as possible.

Before starting any test, the control screen displays peripheral device controls (see Figure 3). Each button links to a particular function on the rig through the PLC.

The user can independently configure each actuator to apply any defined waveform. The system also can apply a load based on a nonstandard waveform to collect actual field data using any acquisition system and send the input to the system. With this feature, we can simulate the actual field conditions of loading and improve the quality of testing and the results generated (see Figure 4).

System synchronization means the user can activate more than one actuator with a set phase difference between them. In live field conditions, the rails and sleepers undergo stress as the wheels pass over them. Therefore, different parts of the rail undergo stress at a particular time lag based on the speed of the train. Both ends of a sleeper undergo stress at the instant the wheel rolls over them. We can simulate these phenomena on the system using the synchronization option (see Figure 5).

System Benefits

The new system we created gives our clients the following benefits:

  • They can now simultaneously test four different samples because each actuator is controlled by an independent output, proportional integral derivative (PID) loop, and feedback channel.
  • They can perform field data simulation by simply providing it as an input file to the system. This reduces the difference in field and lab condition studies. By synchronizing multiple actuators, users also can perform tests under conditions similar to actual field conditions.
  • The real-time controller runs the loops at a fixed interval of 2 ms using a timed While Loop. This guarantees that the PID response for system control and safety is never compromised.
  • With a GUI based on LabVIEW, the user can view all parameters on a single screen and perform comparative studies in real time.

Conclusion

PXI, SCXI, and LabVIEW were the ideal platform for upgrading our client’s decades-old test rig. A key benefit was the shift from a traditional measurement system, which occupied two big panels, to a single tabletop system. We also moved from bulky control panels, plotters, and oscilloscopes to a single intuitive GUI displaying all of the computer-based control facilities and options.

The client uses the new rig to test samples and perform other special trials. They use the results to evaluate the life and durability of the railway systems.

Acknowledgement

This project was done for Indian Railways through a contract from IIT-Kanpur under the guidance of Dr. Sanjay Gupta

Author Information:
Sriram Iyer
Apna Technologies and Solutions Pvt Ltd
No. 1,Coconut Garden Cross,Outer Ring Road,Kalyan Nagar,Post Banaswadi
Bangalore
India
Tel: 9940091584
sriram.iyer@apnatech.com

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