Application Area: Field Testing Challenge: Construct a portable testing system to directly obtain the railway vehicle running stability index from the acceleration signals measured in the field through complex mathematical processing. Application Solution: Construct a portable railway vehicle running stability testing system for field testing using National Instruments' LabVIEW software, SCXI equipment, and a portable computer. Products Used: LabVIEW, SCXI Abstract: This paper introduces a portable railway vehicle running stability tester developed using virtual instrument technology. The test instrument consists of National Instruments' LabVIEW software, SCXI equipment, and a portable computer. Utilizing the powerful mathematical processing capabilities of LabVIEW software, it performs weighted statistical calculations on the spectral density function of the acceleration signals collected in the field to measure the Sperling running stability index of railway vehicles in real time. 1. Running Stability Index Various vibrations generated by railway vehicles during operation affect passenger comfort and the integrity of transported goods. The main technical parameter for measuring the running quality of railway vehicles is the running stability index. Currently, the evaluation index used in China is the Sperling running stability index. The Sperling index, developed based on extensive experiments, is used to evaluate the operational quality of a vehicle and passenger comfort. Operational quality is measured by the vehicle itself, while comfort is also related to passenger sensitivity to vibration. The index used for comfort evaluation is expressed by the following formula: Where j is the peak acceleration (cm/s/s), and F(f) is the frequency weighting correction function, indicating that people have different sensitivities to various vibration frequencies. Within the commonly used frequency range, the vertical and lateral weighting functions F(f) are different. For the frequency weighting function of vertical vibration: The above formula is derived from constant-amplitude vibration at a single frequency. Since vehicle vibration is actually random vibration, the acceleration measured from the vehicle body includes the entire natural frequency of the vehicle. Therefore, it is necessary to group the measured vibration acceleration by frequency, statistically analyze the W value of different accelerations at each frequency, and obtain the overall stability index by the following formula: 2 Virtual Instruments Virtual instruments are a product of the combination of computer systems and instrument systems technology. It leverages the powerful capabilities of computer systems, combined with corresponding hardware, to significantly overcome the limitations of traditional instruments in data processing, display, transmission, and storage, allowing users to "create" their own instruments according to their needs. Compared to traditional instruments, the biggest advantage of virtual instruments is their flexibility. They can easily achieve various measurement and control functions by selecting different hardware configurations and changing the software, making hardware resources reusable. The use of general-purpose hardware and computers reduces system costs, shortens development cycles, and lowers maintenance costs. In a virtual instrument system, hardware is only used to handle signal input and output; software is the core of the entire virtual instrument. National Instruments' LabVIEW is an excellent dataflow graphical virtual instrument software platform with rich signal processing and numerical analysis functions. 3 Portable Stability Measurement Instrument System Composition The portable stability index instrument consists of an accelerometer, an SXCI signal conditioning system, and a portable computer. 3.1 System Hardware Composition The sensor used is a strain gauge accelerometer from Kyowa Corporation of Japan. Based on the requirements of the field test, we selected the National Instruments SCXI system for signal conditioning. Accelerometer signals are connected to the system via SCXI-1321 terminals. The SCXI-1321 terminals provide various bridge connection methods, and the built-in potentiometers allow for bridge balancing. A four-channel isolation amplifier, SCXI-1121, provides four excitation signals with a scan rate of 333 ks/s. The gain of each channel can be adjusted from 1 to 2000 times via jumpers, and low-pass filtering is provided. Since the low-pass filter in the SCXI-1211 module is a three-stage RC filter with a limited frequency setting of 4Hz/10kHz, an eight-channel programmable elliptic filter, SCXI-1141, is used as the system's filtering unit. The software-selectable low-pass filter cutoff frequency is 10Hz-25kHz, and it is an eighth-order elliptic filter with software-programmable gain. Considering the complexity of field testing, a backup DC power supply, SCXI-1382, is provided. The data acquisition card, DAQCard-AI-16E-4, is directly plugged into the PCMCIA interface of the laptop computer. The basic system configuration is a single chassis with 8 channels. Multiple chassis can be cascaded to connect more measurement point signals. Different modules can be inserted into each chassis according to the needs of the test signal measurement points. [align=center] Figure 1 SCXI Test System Structure Diagram[/align] 3.2 System Software Design The test software is entirely developed using LabVIEW. The software algorithm design follows the national standard GB5599-85. [align=center] Figure 2 Software Flowchart[/align] The acceleration signal undergoes spectral analysis to obtain its spectral distribution function. Then, a loop structure is used to calculate the stationarity index for each frequency. For different frequencies, branch structures and formula box nodes are used within the loop for separate processing. Finally, the statistical value obtained from the loop body is used to perform a 10th root operation through the formula box nodes to obtain the final result. The program also performs time-domain statistical operations on the acceleration signal to obtain its average value, maximum (minimum) value, and RMS. [align=center]Figure 3 Vertical stability index calculation program kernel[/align] The program user interface adopts an instrumentation mode. The upper half displays the real-time time history of the signal, the lower left half displays the statistical results of the acceleration signal and the stability index, and the right half is the program control area. [align=center]Figure 4 Program user interface[/align] The calculation results of the test signal can be stored in a data file as needed, and the measurement results can be further processed by the post-processing program. The post-processing program can plot the time history curve of the stability index, the curve of change with speed and road conditions (straight lines, curves), etc. 3.3 Compared with the MKII portable stability index instrument produced by TDM Tape Services Limited in the UK, the stability index instrument developed based on the virtual instrument is only one-third the price of the former, but far exceeds the former in terms of function. Currently, this testing system is configured at Qingdao Sifang Vehicle Research Institute and is widely used in field testing. 4 Conclusion The railway vehicle stability index instrument developed using virtual instrument technology is fully functional, inexpensive, and very suitable for field testing. Developing virtual instruments using National Instruments' LabVIEW and SCXI equipment is a shortcut.