Abstract: This paper presents a virtual instrument testing system for measuring the H-R curve of a sample exhibiting GMR effect using LabVIEW-based virtual instrument technology. By controlling two digital voltmeters via an IEEE 488 bus, the system measures the GMR effect of the sample by utilizing the resistance change trend in a continuously changing magnetic field. The system offers high measurement accuracy, fast speed, and an intuitive and user-friendly testing interface. Keywords: GMR; LABVIEW; virtual instrument; GPIB; automatic testing [b][align=center]A System of Virtual Instrument Based On LabVIEW for Measuring the GMR Effect CHEN chunxian MIN zi jian[/align][/b] Abstract: This paper designs a virtual instrument system based on LabVIEW for measuring the HR curve of a simple object with the GMR effect. The control for the two digital voltage meters is realized by the IEEE488 bus. The GMR characteristic is measured based on the changing dicrection of the simple's resistance in the magnetic field, which has a linear-continuous change. The measurement is of high precision and high speed. The man-machine interface is direct and user-friendly. Keywords: GMR; LABVIEW; virtual instrument; GPIB; automatic testing 1. Introduction The magnetoresistance effect (GMR) of metal thin films has received widespread attention both domestically and internationally due to its high sensitivity, high reliability, and other advantages not possessed by other magnetic sensitive components. Currently, magnetic sensors made using the GMR effect are being used in fields such as magnetic heads, automotive speed measurement, and non-contact switches abroad. In recent years, the GMI (magnetoresistance) effect has been discovered, exhibiting superior performance compared to GMR. The Magneto-Optical Laboratory (a key laboratory of the university) of our Department of Physics is conducting in-depth research in this area. The characteristic measurement of this thin film sample is particularly important. The following is an automated GMR testing system for this thin film sample built using LabVIEW. 2. System Hardware and Software Introduction 2.1 Virtual Instrument Hardware Based on GPIB Bus Because the computer uses a bus standard completely different from the GPIB bus, to enable the computer to act as a GPIB system controller, an interface card connected to the GPIB bus must be inserted into the computer's expansion slot. This paper uses the AT-GPIB/TNT type GPIB interface card from NI (National Instruments). The PC uses this interface card to connect to the GPIB instrument via a GPIB bus cable, becoming a complete GPIB system controller with software support. This system connects two Keithley 2000 digital multimeters (with GPIB interfaces). In addition, the system also has a pair of Helmholtz coils, a scanning power supply (the amplitude changes linearly at a low frequency within ±20V), a constant current source, and a four-pin precision probe (the fixture for the test sample). 2.2 LabVIEW LabVIEW is a graphical programming language, mainly used to develop software for data acquisition, instrument control, and data processing and analysis. It is powerful. At present, this development software is popular in the international testing and measurement and control industry, and it is also widely used in the domestic measurement and control field. Using the graphical programming language of NI's LabVIEW as the development platform, GPIB instruments can be controlled in three ways: (1) Control using the GPIB program library There are many GPIB functions under the InstrumentI/O function sub-template, and many GPIB communication function subroutine modules under the GPIB488-2 sub-template. These modules can call the low-level 488-2 driver software on the working platform. (2) Control using instrument drivers LabVIEW provides drivers for more than 600 GPIB instruments, VXI instruments, and serial port instruments from more than 50 well-known manufacturers around the world. Having software to control a single instrument is not very meaningful; its real significance lies in the ability to call the instrument driver as a subroutine. In this way, the instrument driver library can be used to easily control GPIB instruments. (3) Using the VISA library to implement control VISA (Virtual Instrument Software Architecture) is essentially a general term for an I/O interface software library and its specifications. It includes control operations for various instruments such as GPIB instruments, VXI instruments, and RS232 instruments. All VISA function modules are contained in the VISA submodule of the Instrument I/O function module. The VISA OPEN module is used to establish communication with the specified device; the VISA WRITE module writes the string in the write buffer to the specified device; the VISA READ module reads the data of the specified device; and the VISA CLOSE module closes the communication process of the specified device and releases system resources. This system uses the GPIB program library to implement the control of GPIB instruments. 3. Test System Composition and Principle Based on the principles of virtual instrument systems using the GPIB bus, we established a virtual instrument test system for H-R curves. The computer performs real-time testing on the test object through a GPIB interface card and two Keithley 2000 six-and-a-half-digit digital voltmeters. The sample impedance is measured using a four-port method. Two current leads are connected to a 1mA constant current source, and the other two voltage leads are connected to the Keithley 2000 digital voltmeters. Due to the high input impedance of the voltage measurement circuit, the current drawn is extremely small, thus avoiding the influence of lead and contact resistance on the measurement. The magnetic field strength of the magnetic field where the sample is located is obtained by fitting the curve B=f(U/R), where U is the voltage applied to the Helmholtz coil, R is the coil resistance, and B is the magnetic field strength of the coil under different voltages. 3.1 The system test hardware structure diagram is as follows: This test system studies the resistance change characteristics of a sample in a continuously linearly changing magnetic field, that is, the change of the measured value of digital meter 1 (processed into the sample resistance) with the measured value of digital meter 2 (the magnetic field strength is obtained through a fitted curve). Therefore, an XY waveform recording control is selected to display the measurement results. Software design is crucial to completing the testing function of the virtual instrument. The design of the LabVIEW-based virtual instrument test software includes the design of the front panel and the design of the background graphical control program. The front panel is a graphical user interface that simulates a real instrument, consisting of control, indication, and decoration components. Users can use various icons, such as buttons, switches, XY waveform recording controls, etc., to set input values ​​and observe output quantities. 3.2 Front Panel Design of the Test System LabVIEW establishes a user-friendly human-machine interface through "what you see is what you get" visualization technology, providing a large number of control objects for the testing and process control fields. The instrument control front panel of this system is shown in Figure 1. The main control objects include: [align=center] Figure 1 Front panel design of virtual instrument testing system[/align] GPIB Address: Controls the specified GPIB instrument by setting the address. Instrument functions: Measurement functions of digital meters, including DC current, AC current, DC voltage, AC voltage, two-wire resistance, and four-wire resistance. Number of averages N1 at a certain point: Averages N1 to obtain one element for plotting. N milliseconds to collect one valid point: Sampling time interval. Number of points N: Plotting after collecting N elements. Chart history size: Maximum buffer size of XY waveform records in bytes. Write: Click to start the test. Main display objects: XY chart: H-R curve. STOP: Click to stop sampling and start saving the H-R curve data as a text file to a selectable path. 3.3 The background program corresponding to the front panel is as follows (graphical language): [align=center] [/align] This block diagram fully utilizes the structural framework of program control, such as loops, sequences, and conditions, and flexibly handles the connections between modules. Furthermore, the use of global and local variables greatly simplifies the front panel. The use of SubVI for the buffer chart history size for XY waveform recording in the block diagram fully embodies LabVIEW's modular programming philosophy. After starting the test, valid data is displayed on the XY waveform graph. If the test results are satisfactory, press the STOP button to stop the measurement. When the file save dialog box pops up, enter the file name and save address to save the data for later processing. After the file is saved, the measurement continues. The block diagram program, together with the system hardware, forms a complete virtual instrument testing system, fully embodying the idea that "software is the instrument." 4. Conclusion This paper, based on GPIB bus technology, uses a computer to control a digital voltmeter with a GPIB bus interface through a GPIB interface card. The design of the virtual instrument front panel and background block diagram program was completed in the LabVIEW environment, establishing a virtual instrument testing system for H-R curves. Practical use has proven that the system is reliable and accurate. Compared with software written in traditional languages, the interface is simpler and clearer. This also fully demonstrates the advantages and bright prospects of LABVIEW in the field of automatic testing, especially providing a more intuitive and convenient test platform for the characteristic measurement of giant magnetoresistance. References: [1] Yang Leping et al. LABVIEW Programming and Application. Beijing: Electronic Industry Press, 2001 [2] Liu Junhua et al. Tutorial on Virtual Instrument Graphical Programming Language LABVIEW. Xi'an University of Electronic Science and Technology Press, 2001 [3] KETHLEY2000 User's Manual [4] Guo Zhanshan et al. GPIB Instrument Standard and Test System Composition. Instrument and Meter User, 2002, 9(1): 39-41 [5] http://www.ni.com [6] Xiao Ying, Qi Hanhong. Virtual Instrument Based on GPIB Interface Bus. Microcomputer Information, 2004, 06: 87-89