Virtual instrument technology is a product of the combination of testing technology and computer technology. It is the culmination of the latest technologies in both disciplines, integrating testing theory, instrument principles and technologies, computer interface technology, high-speed bus technology, and graphical software programming technology. A virtual instrument is a measurement and control system composed of computer hardware resources and software for digital analysis and processing, process communication, and a graphical interface. It transforms the way instrument manufacturers define instrument functions into a way where users define them themselves. In other words, traditional testing uses manufacturer-produced instruments whose performance and functions are defined at the factory, and users can only select and use them according to their own requirements and needs. Virtual instruments, on the other hand, allow users to define their own instrument functions by writing software based on a certain hardware foundation, according to their testing needs. The same hardware configuration can be used to develop different instruments. For example, if an instrument displays the waveform of the acquired signal in the time domain on its panel, then the instrument is a virtual oscilloscope; if an FFT transformation is performed on the acquired signal in the program, then the instrument is a virtual spectrum analyzer. The author used LabWindows/CVI to develop a virtual warp tension tester to test the changes in warp tension during the operation of a loom. 1. Warp Tension Sensor During the weaving process, the dynamic tension of the warp yarns has a significant impact on the smoothness of weaving. Excessive tension can easily cause yarn breakage, affecting weaving efficiency; insufficient tension can easily lead to unclear sheds, forming three-skip defects, and making the fabric surface and texture unclear. When the warp yarn passes through the beam, it exerts pressure on the force transmission rods on both sides. The force transmission rods transmit the pressure to the elastic beam, causing strain. The strain gauge converts this strain into a change in resistance, and then the conversion circuit converts the change in resistance into a change in voltage. The voltage value is measured, and the corresponding warp tension can be calculated based on the sensor calibration. 2. Virtual Warp Tension Tester System 2.1 System Structure The structure of the virtual warp tension tester is shown in Figure 2, where the dashed box contains the data acquisition card DAQ (Data Acquisition). Therefore, this testing system consists of a sensor, a data acquisition card, an interface bus, hardware drivers, and developed testing software. The data acquisition card uses the 6024E, and the testing software is developed on the LabWindows/CVI platform, running under the Windows 98 operating system. 2.2 Signal Acquisition Since the relationship between warp tension and spindle rotation angle needs to be measured, three sensors are used. Sensor 1 is a warp tension sensor, which converts the physical signal of warp tension into an electrical signal; Sensor 2 is a photoelectric pulse sensor, used to measure the spindle rotation angle; Sensor 3 is a Hall sensor, using Hall voltage as the measurement trigger signal. The signals output from each sensor are processed by a signal conditioning circuit (such as filtering and amplification), extracting the useful signal to be measured from the mixed signal, and sending it to the data acquisition card. The voltage must be suitable for the voltage range of the data acquisition card, and then sent to the computer for processing via a bus structure. Data acquisition is controlled by software to control the entire DAQ system, including acquiring raw data and analyzing data. The conditioned signal is repeatedly acquired and amplified by a multiplexer under the control of the software-set sampling rate. Then, it is quantized into a digital signal by the sampling and holding and A/D converter units, becoming a signal that the computer can process. The virtual instrument software calculates, analyzes, displays, and stores the results of the test signal. 3. Design of the Virtual Warp Tension Tester 3.1 Panel Structure of the Warp Tension Tester The seven text boxes on the right side of the virtual warp tension tester's panel are set by the user before starting the test, based on actual measurement needs and the connection channel with the data acquisition card. During measurement, turning on the instrument switch allows it to operate; pressing the data acquisition button and waiting a few seconds will display the warp tension waveform on the panel. Saving data involves storing the original measurement data, processed signal data, and data to be provided to the user; reading data involves reading previously measured data and plotting the curve on the instrument panel, which is beneficial for post-analysis; turning off the instrument exits the test state. 3.2 Software of the Virtual Warp Tension Tester The command buttons on the panel, such as data acquisition, turning off the instrument, and saving data, implement their respective functions through callback functions. The data acquisition callback function `caiji` is the key to the program in the entire source code. 4. Application of the Virtual Warp Tension Tester The designed virtual warp tension tester system was used to test a YC-425 air-jet loom, and its warp tension variation curve is shown in Figure 3. As shown in Figure 3, the warp tension changes cyclically with each rotation of the loom spindle. The maximum warp tension occurs near 0°, which is beneficial for weft insertion, while the minimum tension occurs near 280°. Theoretically, the next maximum value should occur at full sheath opening, and generally there are only two peaks. The curve shows two peaks besides the weft insertion point, indicating a tension relief mechanism in the back beam. The reproducibility of the minimum tension, i.e., the warp tension curve, is not very good, indicating that the loom's operating condition is not stable enough. 5. Conclusion Virtual instruments represent the future direction of instrumentation, testing, and control research and development. Using NI's LabWindows/CVI as the software development platform significantly reduces programming difficulty compared to commonly used object-oriented software, resulting in high software development efficiency, a user-friendly interface, powerful functions, and good scalability. The collected data can be used for signal processing in advanced analysis libraries, and can also be fitted to make the obtained test curves conform to actual conditions. In short, virtual instruments have powerful functions, emphasizing that "software is the instrument," replacing hardware with software, making development and debugging easier, and effectively saving costs.