Abstract: This paper introduces the development and concept of virtual instruments. It provides a detailed explanation of the composition, data acquisition structure, and software development application platform of virtual instruments. The differences between virtual instruments and traditional instruments are also presented, and the application of LabVIEW software for virtual instruments is explained using temperature calibration as an example. Keywords: Virtual instrument, Data acquisition, LabVIEW Introduction With the rapid development of control theory and electronic and computer technologies, many new theories, technologies, and concepts have emerged in the field of detection technology to meet the needs of scientific research and production. Virtual instruments have thus come into being. A virtual instrument (VIRTUAL INSTRUMENT) refers to a testing technology platform that combines a computer with functional hardware (dedicated hardware for signal acquisition, conversion, and conditioning) through an application program. This integrates the powerful computing, storage, and communication capabilities of the computer with the measurement and conversion capabilities of the functional hardware, forming a multifunctional, high-precision, flexibly combinable testing technology platform with communication functions. In electronic measurement, it can replace traditional oscilloscopes, logic analyzers, signal generators, and spectrum analyzers. In industrial control systems, all computer-centric automation devices can be categorized as virtual instruments, replacing conventional controllers, handheld devices, indicators, and alarms typically installed in control rooms. When using virtual instruments, users can operate the computer through a user-friendly interface on the screen, just like operating custom-defined traditional instruments, thus completing functions such as acquisition, analysis, judgment, adjustment, and storage of measured quantities. I. The Development History of Virtual Instruments The development and application of virtual instrument technology originated from LabVIEW software designed by National Instruments (NI) in 1986. This is a graphical software platform for development, debugging, and operation. It realized NI's "software as instrument" concept. The development of virtual instruments can be roughly divided into three stages. The first stage was using computers to enhance the functions of traditional instruments. Due to the establishment of the General Purpose Interface Bus (GPIB) standard, communication between computers and external instruments became possible. Therefore, by connecting traditional instruments to a computer via a serial port, the computer could control the instruments. The second stage saw significant technological advancements in functional hardware. Firstly, the emergence of plug-in PC-DAQ data acquisition (PLUG-IN PC-DAQ) and secondly, the establishment of the VXI instrument bus standard. These new technologies laid the foundation for virtual instrument hardware. The third stage formed the basic framework of the virtual instrument architecture. This was primarily due to the adoption of object-oriented programming techniques to build several virtual instrument platforms, which gradually became standard software development tools. Due to the rapid development of virtual instrument technology, these three stages of development occurred almost simultaneously. II. Structure and Hardware of Virtual Instruments A virtual instrument generally consists of three main functional components: a computer, functional hardware modules, and application software. These components exchange data via a standard bus. The composition of a virtual instrument is shown in Figure 1: [align=center] Figure 1: Composition of a Virtual Instrument[/align] Commonly used virtual instrument systems typically include signal-conditioned data acquisition systems; test systems with a General Purpose Instrument Bus (GPIB); VXI instrument test systems; and any combination of these three. A typical data acquisition virtual instrument system consists of four parts: signal acquisition, signal conditioning, data acquisition, and data processing, as shown in Figure 2. [align=center]Figure 2: Block Diagram of a Virtual Instrument System for Data Acquisition[/align] A good data acquisition system should not only possess high performance and high reliability, but also provide comprehensive drivers and a universal high-level language interface. Only in this way can it provide users with the greatest convenience in quickly building their own application systems. Currently, due to the application of multilayer circuit board technology, programmable amplifier technology, system timing controller technology, high-speed data acquisition double buffering technology, and high-tech technologies such as interrupts and DMA for high-speed data transmission, new data acquisition cards have achieved very high standards in various performance indicators. III. Software Development Platform for Virtual Instruments Software is the core of virtual instruments, and currently, the main software development platforms include: National Instruments (NI)'s LAB VIEW, LAB WINDOWS/CVI, and HP's VEE. Virtual instruments fully conform to the internationally popular trend of "hardware becoming software," and are therefore also called "software instruments." NI not only provides users with various hardware components that constitute virtual instrument systems, such as data acquisition boards, various GPIB instruments, and VXI instruments, but also provides a compiled graphical programming software, LAB VIEW. It simplifies complex and cumbersome language programming by allowing users to select graphical functions using menus or icons, and then connect the function diagrams with lines to complete the programming work. Users with C programming experience can use NI's other virtual instrument software development platform language, LAB WINDOWS/CVI, which simplifies program development and improves programming speed. In a virtual instrument system, hardware is only used to handle signal input and output; software is the key to the entire system. All system functions are primarily implemented by software. Any user can easily modify the software to change, add, or remove system functions and scale, building their own general or specialized testing platform. IV. Differences from Traditional Instruments The difference between virtual instruments and traditional instruments lies in their functionality. Traditional instruments have limited functionality and are defined by the manufacturer, resulting in closed systems, fixed functions, and low scalability. Due to the limited information, manual reading and report generation are generally required. Virtual instruments, on the other hand, allow users to fully define and configure their functions through programming, creating customized testing systems to meet their specific needs. Furthermore, it can implement multimedia operator commands; time stamping and measurement annotation; measurement correlation and trend analysis, and other functions. Most importantly, it can achieve programmable fully automatic testing and automatic result analysis. It also has advantages in performance-price ratio. Virtual instruments can be widely used in scientific research and engineering fields such as engineering surveying, mineral exploration, biomedicine, vibration analysis, and fault diagnosis. Computer monitoring systems widely used in process industries can also be considered virtual instruments. V. Temperature Calibration System Based on Virtual Instruments Unlike conventional temperature calibration systems, the core of a virtual instrument system is a computer. Its functional hardware consists of a high-precision digital multimeter with a GPIB interface and a temperature controller. The heating control circuit uses a high-voltage thyristor element and trigger circuit, which receives command signals from the temperature controller to adjust the voltage value of the heating element in the calibration furnace. If an electronic automatic switching switch is added, automatic calibration of multiple thermocouple thermometers in the same furnace can be achieved. The virtual instrument's software utilizes LabVIEW, employing graphical C language programming technology to simplify complex and time-consuming software programming into menu prompts and icon-based connections. The virtual instrument's functions include: Ø Temperature control of the calibration furnace and confirmation of the calibration equilibrium point. The computer receives temperature field signals and outputs control commands through data communication with a digital multimeter and temperature controller. It sets constant temperature control setpoints, over-limit alarm values, etc., on the user interface displayed on the screen according to different requirements and provides indicators for the stability criteria of the equilibrium temperature point. The computer then operates according to certain adjustment rules and discrimination formulas. Ø At the temperature equilibrium point, effective temperature data is collected through an electronic automatic switch and a digital multimeter. The computer obtains this data via a bus and performs preprocessing, removing values ​​with negligible and systematic errors. Ø Real-time display of the temperature control curve and calibration comparison curve. Calculations and uncertainty assessments are performed on the effective data, and finally, a calibration report is output. All raw data and final results are entered into a database for storage. A virtual instrument-based temperature calibration system can fully meet the requirements for calibration and verification of different thermometers using different standard equipment in a temperature calibration laboratory. It utilizes a single hardware device and different software to meet the calibration requirements for various temperatures and temperature ranges, featuring high precision, high efficiency, and full automation. It represents the future direction of modern temperature metrology laboratory calibration equipment. VI. Summary A virtual instrument is essentially a computer testing system that can perform all the functions of a traditional instrument. It maintains the human-machine interface and operation mode of traditional instruments, except that the instrument panel is displayed on a screen, button operations are performed by clicking a mouse, and alarm lights on the screen can also flash. However, virtual instruments are more flexible and convenient, have more powerful functions, and can be configured into any testing system as needed. Remote networking of virtual instruments is also a development direction; measurement signals transmitted through a network can achieve resource sharing, remote or off-site control, data acquisition, and fault monitoring.