1. Introduction Currently, most thermal expansion meter testing systems in China are developed using VC++, which involves complex programming and cumbersome inter-instrument communication, requiring significant time investment. Virtual instruments, proposed by National Instruments (NI), are a comprehensive testing technology. They combine several common basic instrument hardware modules on a computer, using software to create various functional instruments and systems. Virtual instrument technology utilizes LabVIEW for development. LabVIEW system development shortens the development time of complex programs, solves testing problems more quickly and economically, and its user-friendly interface has led to its increasing application in the testing field. It integrates various communication bus standards such as PCI, DAQ, GPIB, PXI, VXI, RS-232, and RS-485, possessing powerful external interface capabilities and easily facilitating inter-software communication. Using LabVIEW for programming saves time and costs. Due to the simplicity and speed of virtual instrument development, this paper develops a thermal expansion meter testing system using LabVIEW software. The system has a user-friendly interface, is easy to operate, and facilitates measurement. Utilizing its powerful interface communication capabilities, it achieves signal measurement, data acquisition, analysis, processing, storage, and display through VISA communication. 2. Overall Design of Thermal Expansion Testing Scheme The testing system places the material to be tested inside a heating furnace. As the temperature rises, the material expands due to heat. The expansion amount is transmitted to a displacement sensor via a push rod. The displacement sensor measures the displacement of the material due to thermal expansion. As the furnace temperature rises, the system transmits the temperature signal and the changing displacement signal to a PC in real time through data acquisition and processing. The thermal expansion coefficient of the material is calculated using the thermal expansion formula. This is the basic principle of measuring thermal expansion. The overall structural block diagram of the test is shown in Figure 1. First, the displacement and temperature sensors are used as signal inputs. The signals are amplified and filtered by a conditioning module, and then transmitted to the PC after data acquisition to obtain the current temperature and displacement signals. A PID control algorithm is used to control the temperature of the electric heating furnace, causing the temperature to rise linearly. With this linear temperature change, the thermal expansion curve can be obtained by simultaneously measuring the material's expansion. Finally, the expansion coefficient within any temperature range can be obtained through the expansion curve and the test data. [align=center]System Overall Structure Diagram[/align] 3 Basic Hardware for Thermal Expansion Test 3.1 Temperature Sensor Temperature detection uses nickel-chromium-nickel-silicon thermocouples (Type K thermocouples). Type K thermocouples have the characteristics of good linearity, large thermoelectric potential, high sensitivity, good stability and uniformity. They can measure a maximum temperature of 1300℃, and are small and robust, suitable for use in harsh environments. However, they can only produce millivolt (mV) level outputs, so precise amplification is required for further processing. The voltage-temperature curve of thermocouples usually uses 0℃ as a reference point, but the actual temperature is usually not 0℃. Therefore, cold junction compensation (CJC) technology is generally required. 3.2 Displacement Sensor The displacement sensor is a key component for displacement measurement, and the sensor accuracy affects the measurement accuracy. This system uses a variable linear differential transformer (LVDT), which is a sensor that converts position signals into electrical signals. Its output is proportional to the movable magnetic core. The magnetic core moves linearly within the transformer formed by the central primary coil and the two outer secondary coils. The primary coil is energized with voltage, and the secondary coils are wound in opposite directions. Therefore, when the magnetic core is in the middle position, the net output voltage should be zero. However, due to the inconsistency between the two secondary coil windings and the leakage inductance, the output will not reach a true zero value. This problem is generally solved using a signal conditioning circuit that subtracts the absolute values ​​of the two output voltages. Using this method, positive and negative changes around the center position can be measured. The displacement sensor performance specifications are: measurement range ±2.5mm, measurement voltage ±12VDC, output signal ±5VDC, nonlinearity of 0.05%, and temperature coefficient of 0.05%. 3.3 The signal conditioning module uses ADAM-4018 as the temperature sensor signal conditioning module. The ADAM-4018 is a 16-bit, 8-channel analog input module that can directly acquire thermocouple signals. It can be configured to correspond to different thermocouple signals. This paper uses three analog input channels to test the temperature of the furnace heating element, the temperature near the measured material, and the ambient temperature of the displacement sensor. The ADAM-4018 uses an RS-485 communication interface, and all channels offer programmable input ranges. It can measure various analog signals, such as thermocouple signals, mV, V, and mA signals, and can convert between different signal ranges. It includes a cold junction compensation circuit for compensation and comes with Advantech Device Manager software, eliminating the need for software-based conversion of analog signals to K-type thermocouple temperature signals. It can directly map the analog voltage to the set thermocouple type to generate a temperature signal output, avoiding cumbersome interpolation methods or high-order polynomial simulations. 3.4 Data Acquisition Card For convenient signal acquisition, this system uses the PCI-9118HG with a parallel 32-bit transmission bus as a plug-in data acquisition system. This is a data acquisition card with 16 single-ended/8 differential inputs and 2 analog outputs. The output signal is conditioned to output an analog voltage, which controls the voltage regulation module to control the output power of the heating element. 4 Software Design for Thermal Expansion Testing LabVIEW is a virtual instrument software development platform based on a graphical programming language. It has powerful function modules and communication protocols, greatly reducing the software development cycle, especially in test and control systems, where it facilitates data acquisition and analysis. This system divides the thermal expansion test into two parts: a measurement module and an analysis module. The software functional structure of the thermal expansion meter testing system is shown in Figure 2. [align=center] Software Functional Structure Diagram of the Thermal Expansion Meter Testing System[/align] 4.1 Signal Acquisition of the Differential Transformer The PCI-9118HG is used for signal acquisition. Due to its fast transmission speed, it can quickly transmit the displacement changes caused by thermal expansion, showing good response, as shown in Figure 3. Since the system does not use an NI data acquisition card, the PCI-9118HG does not support certain NI DAQmx controls, so DAQ acquisition is used for data signal acquisition. The acquisition card converts the input analog signal into a corresponding +2.5mm signal output. A formula node is added in Figure 3 for scaling transformation to convert the signal into the required signal range. 4.2 Thermocouple signal acquisition and VISA serial communication: The ADAM-4018 is used for thermocouple signal acquisition. All channels of the ADAM-4018M provide programmable input ranges. Its built-in software, Advantech Device Manager, can condition the input signal and also allows for user-defined CJC compensation correction. The ADAM-4018 is an RS-485 serial communication module. The signal is transmitted to the PC through the RS485/RS232 adapter. The software allows sending commands to control the module's output. Next, commands will be sent to send the data acquired in the ADAM-4018 to the LabVIEW software interface via VISA communication. First, the input signal needs to be configured, such as baud rate, data bits, stop bits, and whether parity is required. This must be the same as the settings on the PC. Next, VISA is enabled, and code is written to the instrument via VISA before being disabled. A command is sent to the ADAM-4018 module to transmit the processed signal via serial port to LabVIEW. The thermocouple temperature signal is then output in LabVIEW without needing to run the Advantech Levier Manager software separately. Figure 4 shows the serial port program for VISA communication. #01 in the write buffer transmits the data output from address 01 to LabVIEW. When writing to the write buffer, its attribute must be changed to allow code writing; otherwise, no results will be output. After data transmission, the ADAM-4018 returns a string containing symbols such as > and +, which must be converted to data for output. The acquired temperature signals are the furnace temperature, the bar temperature, and the ambient temperature near the differential transformer. Only the furnace temperature is controllable. 4.3 PID Control The PID control front panel is shown in Figure 5. The control effect is tuned by adjusting the PID parameters. As a classic control algorithm, PID control is frequently used in temperature control and has good control performance. This system collects and reads temperature signals into the PID module, which then controls the heating output power to ensure the temperature approaches the target temperature with a good response. The specific values ​​of the P, I, and D parameters are adjusted according to the actual situation. The PID module includes parameter self-tuning, which can analyze noise levels based on the current environmental conditions and automatically set the PID parameters. 4.4 Data Storage and Analysis The data storage section is shown in Figure 6. After starting the program, a prompt will appear to save a file, with the default name being file1.txt. The user can modify this file. The system will record the time, temperature, and corresponding displacement values ​​according to the sampling rate. The front panel for data analysis is shown in Figure 7. By setting temperature ranges, the user can access the saved file. The system will automatically retrieve the displacement corresponding to the user-input temperature value from the stored data file. Based on the material's length and temperature change, the displacement is calculated using the thermal expansion coefficient formula αm = (L2 - L1) / [L0(t2 - t1)] = (ΔL / Lo) / Δt(t1 ). Conclusion and Outlook: This system constructs a thermal expansion testing system, using NI's LabVIEW software for programming. Leveraging the powerful communication capabilities of virtual instruments, it achieves data acquisition and VISA serial communication. The thermal expansion testing interface is user-friendly and easy to operate, enabling convenient and flexible measurement of the expansion coefficient of the tested object. It also has data analysis capabilities, automatically calculating the thermal expansion coefficient for the required temperature range by calling the stored data file. The construction of this system has positive significance for thermal expansion testing and material performance research.