the instrument category, Furthermore,we belleve that remote operation of an instrument, (e.g. digital voltmeter),interfaced to a computer does not necessarily make it a Virtual ,a instrument.There are usually two operating modes available: remote or local, as in the IEEE 一 488 system. Instead of a DAQ board, a virtual instrument can incorporate stand?alone devices, inlerfaced to the chosen PC bus,via cable and providing the same functionality as a DAQ board.Such data acquisition devices are often delivered under brand names,such as WaveBook(designed for PC operation only),or LogBook(operating as a PC peripheral or Stand-alone)from IOtech. In addition to a computer, a DAQ board and a connector,as shown in Figure
  3.
  12,virtual instrument-based measurement system hardware may and usually does include additional components: A conditioner card to match the input levels to the DAQ board's input range; A conditoner card for temperature, pressure, or displacement sensors; Expansion cards to provide additional analog inputs, high-volatage inputs, or lowvoltage inptus; An optically isolated analog/digital input card; A BNC coax input card; A low pass filter card or other cards. Figurse
  3.13 show a measurement system with a virtual instrument and additional cards. An adapter board is used to connect more than one conditioner card or expansion card to the DAQ board input. A virtual instrument-based measurement system, as show in Fiure
  3.13, can be used to measure and process various phsical qualities(e.g., Temperature, pressure, or displacement) and electrical signals spanning a wide range. means of By limited technical resources. The measurement performance of such a system depends not only on specifications of the boards used, but also on the characteristics.of the system's application software.By using signal filtration and averaging,statistical calculations.and other procedures,the quality of final measurement results may be significantly improved. 2 Programming of Measurement Systems and Virtual Instruments Software is an indispensable part of a computer?based measurement system and of a virtual instrument.High?level graphical programming languages,known as measurement system development environments,provide extensive software development capabilities and Make it easy to program a measurement system or a virtual instrument.Furthermore.the major instrumentation manufacturers offer proprietary programmiug languages (environments).such as LabVIEW or LabWindows from National Instruments,VEE (formerly HP VEE)from Agilent Technologies.and TestPoint from Keithlev. All these languages have a number of advantages but they are rather expensive.The price is several thousand dollars for a basic version.Graphically rich general?purpose programming languages,such as Visual C++or Visual Basic,and commands from the list 0f Standard Commands for Programmable Inst ruments (SCPI), can also be used to develop virtual Instrument software. LabVIEW, LabWindows, VEE.and TestPoint all come with libraries Of drivers for specific instruments, such as multimeters, digi tal oscilloscopes,analyzers, DAQ boards,and generators. Insturmentation manufacturers ensure that the drivers for
Their instruments are available in the the libraries of most popular development environment。 The deveIopment issues arc so elaborate that each package comes with a two-volume documenttation and requires a comprehensiVe manual. 3 Soflware Development in the LabVIEW Environment
The Principles of developing measurement system(or virtual instrument)software in a develoPment enbironment are described briefly in the example of using LabVIEW to build a virtual instrument(a spectrunl analyzer)with a DAQ board. Each software developed in LabVIEW consists of two main parts: .The Graphical User Interface(GUI),which provides an integrated front paneI of the instruments making up the measurement system or Virtual instrument Figure3?14 shows a sample front panel of a virtual spectrum analyzer in LabVIEW
  7; . The functional diagram(block diagram), which maps the function block icons and relations between these function blocks of the virtual instrument. See Figure 3?1 5 for a block diagram of a spectrum analyzer.This block diagram is the source code of the virtual instrument control program. To write a program in LabVIEW,a block diagram is created by dragging and dropping Object icons function icons fmm the All Functions palette,as shown in Figurc
  3.
  16, and the All Controls palette, as shown in Figure
  3.1
  7,onlo the panel. Many of these functions in the diagram can be expanded on multiple levels to set their Required properties. Surrounding the selected objects with a loop forces a conditional execution or repetition of the selected operations, corresponding to the effect of if or while instructions known from other programming languages . Instrument drivers which are necessary for the program to run should be stored in the LabVIEW driver library.Once the icon of a specific instrument is placed on the system diagram, as shown in Figure
  3.15, the program will use that that instrument's driver at runtime. Figure
  3.48 shows the expanded view of program menus that must be insret the HP34401A digital mulatimeter dirver form Hewlett-Packard into the system. Creating control programs in VEE or TestPoint is similar to the technique described earlier.
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