Abstract: Virtual instruments represent the future development direction. This paper introduces the basic knowledge and current status of virtual instruments and describes the specific process of developing a hydraulic pump source measurement and control system using PXI bus technology. The system uses a variable frequency speed control system and a proportional throttle valve as the hydraulic pump source loading system, and LabVIEW software as the development platform, which can effectively complete various experimental requirements. Keywords: Virtual instrument, PXI bus, LabVIEW, Measurement and control system 1. Introduction The development of science and technology is closely linked to the development of testing technology. Many major scientific achievements are obtained through advanced experimental methods. Therefore, possessing advanced scientific testing capabilities is a hallmark of scientific and technological modernization, and this is also true for the hydraulic industry. This paper studies a hydraulic measurement and control system based on the PXI bus. It aims to overcome the shortcomings of traditional testing systems currently used in industry by seeking reasonable testing schemes, better complete the performance testing of hydraulic systems, and promote the application of virtual instruments in automatic measurement and control systems. 2. Current Status and Development Direction of Virtual Instruments 2.1 Introduction to Virtual Instruments Virtual instruments are a new type of instrument that has emerged with the development of computer technology, modern measurement technology and electronic instrument technology. They are based on general-purpose computers and equipped with specially designed hardware (such as data acquisition cards, VXI/PXI chassis, etc.) and software. They have both operation panels similar to traditional instruments and special functions that traditional instruments do not have. It is a computer instrument system that uses I/O interface devices to complete signal acquisition, measurement and conditioning; uses the powerful software function of computers to realize the operation, analysis and processing of signal data; uses the display function of the monitor to simulate the control panel and express the output test results in various forms; thereby completing various test functions. Currently, the most commonly used virtual instruments are data acquisition systems, GPIB control systems, VXI, PXI instrument systems and any combination of these [1]. 2.2 Characteristics of PXI Bus The PXI bus is based on CompactPCI and is extended from the PCI bus with open characteristics (proposed by NI in 1997). PXI is similar in architecture to VXI, but it has lower equipment costs, faster operating speed, and a more compact size. It conforms to industry standards and fully leverages the advantages of the PCI bus in terms of mechanical, electrical, and software characteristics. Currently, both PCI bus-based hardware and software can be applied to PXI systems, resulting in excellent compatibility. Furthermore, PXI offers high scalability, with 8 expansion slots, which can be expanded to 256 slots using PCI-PCI bridges. The current PXI bus transmission rate has reached 132 Mbps, with a maximum of 500 Mbps, the highest transmission rate currently available. The combination of the cost-effectiveness of desktop PCs and the expansion advantages of the PCI bus in the instrumentation field will form the main virtual instrument platform of the future. 2.3 Characteristics of Virtual Instruments *Funding Project: National Natural Science Foundation of China (60572001) Research on Capacitive Weighing Sensors and Their Application in Vehicle Weighing The concept of virtual instruments represents a significant breakthrough from the traditional instrument concept, a product of the integration of computers and instrumentation. Compared to traditional instruments, virtual instruments offer high flexibility, allowing users to define their functions through software development. Virtual instruments possess characteristics such as openness, modularity, reusability, and interchangeability in both hardware and software. Software is the core of virtual instruments, enabling users to define instrument functions according to their needs. 2.4 Applications of Virtual Instruments Virtual instrument technology is widely used in developed countries, and my country has also attached great importance to its research and development in this area in recent years. Tsinghua University's Department of Automotive Engineering has built an automotive engine testing system using virtual instrument technology for measuring the power and load characteristics of automotive engines. The State Key Laboratory of Fluid Transmission and Control at Zhejiang University has developed the V-SDTH general-purpose computer-aided testing system for hydraulic components, enabling stable dynamic testing of various valves. Beihang University has developed a multi-equipment laboratory test management system for a institute under the Second Academy of Aerospace Science and Technology. These applications have all achieved good practical results. 3. Application of Virtual Instrument Technology on Hydraulic Test Bench 3.1 Design Requirements of the Hydraulic Pump Test Bench Measurement and Control System The test items specified in Article 5.3 "Factory Test" of JB/T 7044-93 "Test Method for Hydraulic Axial Piston Pumps" include "Displacement Verification Test," "Volume Efficiency Test," "Variable Characteristic Test," and "External Leakage Inspection Test," requiring the achievement of Class B test accuracy as specified in Article 3.8.1 of GB/T 7936-87 "Method for Determining the No-Load Displacement of Hydraulic Pumps and Motors" and Class B measurement accuracy as specified in Article 4.5 of JB/T 7044-93 "Test Method for Hydraulic Axial Piston Pumps." 3.2 Technical Route and Test Scheme To achieve the testing of hydraulic pump performance parameters, it is necessary to test and control parameters such as pressure, flow rate, temperature, torque, and speed of the hydraulic pump source system. Meanwhile, to ensure the safe operation of the system, the system has alarm functions for overpressure, overtemperature, oil filter contamination, and low liquid level. Simultaneously with the alarm, relevant safety measures are taken, such as unloading and shutting down the pump. See Figure 2 for the system loading device diagram. 3.3 Measurement and Control System 1. Hardware and Software Configuration This test system requires high precision, high reliability, and real-time performance. Considering several hardware configuration schemes for virtual instruments, this measurement and control system adopts the NI PXI bus test system. It uses an NI-1042 chassis with a built-in PXI-86 controller. It uses the Windows XP operating system and installs LabVIEW virtual instrument software to complete data acquisition, calculation, control, storage, plotting, analysis, and printing. This system uses the NI M-series NI6259 multifunction card and the Siemens S7-200 programmable controller. See Figure 3 for the overall structure diagram of the test system. [align=center] Figure 3 Overall Structure Diagram of the Test System[/align] 2. Signal Conditioning Each sensor outputs a standard current of 4-20mA. These signals are connected in series with a 500Ω precision resistor to convert them into the voltage signals required by the multifunction card. Important signals, such as the output signal controlling the frequency converter and the signal controlling the pressure of the throttle valve, are isolated by the signal isolation module; I/O points that need to be connected to both the computer and the PLC are isolated by the opto-isolation module. 3. Measurement and control software [2][3] The measurement and control system software is the key part of the test bench system. It acquires information through communication equipment and displays the required information to the user. The tasks to be completed by the measurement and control system software include: data acquisition, parameter setting, data analysis and processing, parameter display, data storage, signal alarm, report printing, etc. Considering the advantages of LabVIEW's user interface style, high development efficiency, ease of use and development extension, this system uses LabVIEW software as the development platform. 1. System settings module. Includes two sub-menu items: self-test and hardware settings. Self-test is performed automatically after the connection is completed and the user confirms. Hardware settings allow the user to click the mouse to select specific switches to observe their working status. 2. Project selection and testing module. This is the core module of the measurement and control system. It calls up relevant test information, manipulates the relevant components of the system to establish and convert various test data, and collects data. Finally, it obtains the test results of each test point through data processing algorithms. 3. Result module processing. It is responsible for generating, plotting, displaying, printing and storing the standardized result report files of each specific object. It is a later work, which performs non-real-time reproduction and processing of the sampled signal, such as spectrum analysis and graphic processing. 4. Help and exit module. The help module provides online help to users and guides them to operate correctly; the exit module is for the system to exit the test. The main interface of the system test is shown in Figure 4: [align=center] Figure 4 Main interface of system test[/align] The torque change curve of the test result with the oil outlet pressure is shown in Figure 5: [align=center] Figure 5 Test graph of pressure and torque change[/align] 4. Summary The innovation of this paper is to effectively combine computer control, detection technology and hydraulic control technology, which can control and detect various parameters in the system in real time. The LabVIEW virtual instrument software is used as the development platform, which has the advantages of short development cycle, saving research and development costs and user-friendly interface. After actual operation, the system has the characteristics of high detection accuracy, stable and reliable operation, and convenient operation. 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