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【正文】 ion card. It has interior program warehouse providing various ways of connection. It could connect with exterior program and software system through DDLs and ActiveX, to carry out the tasks that can’t be finished by or difficult performed by itself. After all, LabVIEW’s unitary function is amazing.II. Design of data acquisition virtual instrumentA Testing Principle of pressure signalThe testing of instantaneous pressure in our research is based on PCDAQ system. First of all, pressure in gun barrel is transformed into electric signal by pressure sensor. A charge amplifier magnifies the signal. At last, the magnified signal is transferred into puter through data acquisition card fixing in PC. DLL makes data acquisition card and LabVIEW connect together for further displaying and processing. B Hardware of data acquisition virtual instrument Hardware of data acquisition virtual instrument mainly contains two parts: First part is puter, generally one PC or workstation. It is a core of hardware platform. Second is I/O interface instruments. Its main works are signal acquisition, magnification of signal and analogdigital transformation. The selection of I/O interface instruments depends on actual needs. I/O interface instruments include data acquisition card, GPIB, VXI and serial interface. PCDAQ system is a virtual instruments system and its main hardware contains acquisition adjustmen circuit and personal puter. PCDAQ has PCI, ISAor general puter bus. Once insert data acquisition card into empty rabbet of the puter, the system is ready to work. The software LabVIEW makes instruments transport data into puter and the puter records and processes the data. Also PCDAQ test system is the cheapest in numerous virtual test systems. How to make up of a PCDAQ virtual test system? First insert data acquisition card into empty rabbet of puter (LabVIEW has all kinds of data acquisition card driver to make DAQ work properly)。 Second, A/D transducer transports digital/analog signal into puter。 Third, puter analyzes and displays the digital signal. Moreover, it can do feedback control through D/A transducer. The data acquisition system is multicenter and multiparameter structure. The main tasks of data acquisition and processing system are catching and recording signal. First, pressure signal should be transformed into electric voltage or current signal. Second, adjust signal through adjustment circuit. At last, the signal is transported into puter through data acquisition card.Interface of A/D transducer and puter are integrated in data acquisition card PIOMIO16E1 produced by National Instrument. Data acquisition card PIOMIO16E1 has 16 analog signal channels that could carry out 16singlechannel input and 8 pairs of difference input. So it could acquire several groups of signals simultaneously. In this virtual instrument system, singlechannel input is used. Sampling precision is 12 bit in A/D transducer. Then, the signal is transported into puter and goes on with next step.C Software platform of virtual instrument Data acquisition virtual instrument system mainly used in processing control, signal transportation and display acquired data. Furthermore, it could save and print files automatically. The data acquisition program was written with software LabVIEW. The system contains a main VI and 5 sub VI (In LabVIEW, a relatively independent program module that has special function is called a VI). “.” is a window of data acquisition virtual instrument system. ① idstsdemo. VI is a data acquisition VI and it is the main VI. It controls data acquisition, signal transportation and data display. Moreover, it can save and print files automatically. FIG. 1 A window of data acquisition virtual instrument system② global. VI is global parameter VI, and it defines global parameters. ③ sysinit. VI is system initiation VI, and it memorizes initial parameters.④ iocontrol. VI is I/O control VI, and it has 16 channels of input/output. ⑤ setpara. VI is parameter setting VI, and it is used to set parameters such as sampling hits, length, delaying and channels. ⑥ transducer. VI is electric amplifier VI, and it is used to setting amplifier and transducer parameters.(1) Starting acquiring Once, start data acquisition, all of the channel start acquiring data. After it starts, system can’t read data immediately and system must have a state function to test acquired signal, because acquiring would last a period of time and system should supervise data acquisition card’s state. “.”is acquiring procedure program that controls data acquisition procedure. (2) Displaying data intelligently The system could display data in graph and curve that make change trend of data more clear. Moreover, it provides various graphic widgets to magnify and deflate graph and show coordinate of special point. All of them make system’s graph function stronger.(3) Saving and printing files After acquiring data successfully, the system can create four kinds of data files, including text file, binary file, wave data file and wave graph file. (4) Setting parameter Parameters should be set before acquiring starts. It contain: I/O control parameter, acquiring process parameter, electric amplifier parameter and so on.D Communication of LabVIEW and data acquisition cardThere are two ways to realize munication between LabVIEW and data acquisition card: CIN (code interface node) and DLL (dynamic link library). In LabVIEW, CIN is a code interfacenode that connects with traditional program language C. CIN could create a template program and user addscorresponding source code in the template program that will later be piled into .lsb file under IDE environment of . Then, load the .lsb file in the CIN node of LabVIEW and user can make use of it directly. When LabVIEW run the CIN node, LabVIEW uses the executive .lsb file directly, and transmits data into the executive file and output result. Acq