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Instrumentation programming – doing it the graphical way In the past, PCs have been used to control data acquisi­tion and test equipment via programs written in the conventional way, with hundreds or thousands of lines of text based code. This article discusses LabVIEW – software based on graphical program­ming. By JACK BARBER The introduction in 1986 of Lab­ VIEW (Laboratory Virtual Instrument Engineering Workbench) for the Macintosh revolution­ ised PC-based instrumentation with the concept of graphical programming – developing block diagrams rather than writing conventional, text-based code. Lab­ VIEW was the first graphical program- ming language used to integrate several popular classes of instrumentation hardware for test and measurement applications. In 1992, National Instruments announced LabVIEW version 2.5 for Sun SPARCstations while version 3.0, introduced in 1993, made graphical instrumentation applications complete­ ly portable between Macintoshes, Win­dows PCs or Sun SPARCstations. This article explains the benefits of graphical program­ming with LabVIEW and the characteristics and features of LabVIEW that differentiate it from other products that appear to have a similar look and feel. Graphical programming Graphical programming offers the ability to create software applications to those who otherwise do not have the time or skills to program using conventional text languages. Graphical programming lets the user draw a diagram or a picture to explain a process or algorithm. A user can easily scan a picture of a graphical program for relevant features, data flow structure and complex relationships that would otherwise be hidden in the code of a text-based program. Graphical programming can be tailored for a particular application area. By supplying the user with familiar tools and terminology, the software package serves as an enhancement rather than a hindrance to the application. A LabVIEW program or subpro­gram is like an instrument with front-panel controls. The “in­strument” measures inputs and displays outputs. This instrument also has internal circuits. G, the “language” in LabVIEW, gives users the ability to draw the schematic, so to speak, for these circuits. These software emulations of hardware instruments are therefore called “virtual instruments,” or VIs. Graphics vs icons Fig.1: system developers use pull-down and pop-up menus to equip the front panel with indicators and controls. The front panel serves as the graphical user interface during program execution. 22  Silicon Chip Today, several software products use icons for visual representations however few of these are true graph- ical program­ming systems. Most are menu-driven systems where each icon repre­sents a function and contains a list of options. Users connect these icons to specify an action. Icon-based systems are typical­ ly limited by a small set of functions, options and ways in which users can connect the icons. However, such programs may well satisfy users who have simple application requirements that will not become more demanding in the future. With Lab VIEW ’s graphical programming approach, the novice can quickly assemble simple programs such as those typically created with menu-driven packages. However, more experienced users will find LabVIEW also offers a good alternative to con­ven­tional text-based languages such as BASIC or C. Like convention­ al programming systems, LabVIEW incorporates features such as hierarchy, execution control, programming structures and also a compiler. Virtual instrument Before selecting software, it is important to consider how you want your system to present data. Due to limited space on the screen, combining the GUI elements with the functional elements in a diagram (as some software products do) is impractical for complex applications. A LabVIEW VI has separate panels and a diagram optimised for operating and programming, respectively. On the front panel, users arrange the controls and indicators in a logical order, add background pictures and create custom controls to add context to the GUI (Graphical User Interface). In the diagram, small graphi­cal equivalents of the GUI elements save space and make it easier to construct a block diagram. Fig.2: Temperature System VI has a While Loop that contains a For Loop (which acquires a group of temperature readings) and a Case Structure (which determines if the data is to be analysed.) You can click a switch, move a slider, tweak a knob, or type a value on the front panel to interactively control the system during execution. Meanwhile, the indicators provide feed­back and results. LabVIEW can store the data by printing the front panel or by saving it as a picture file. LabVIEW’s block diagram defines what the virtual instrument (VI) does. The block diagram contains terminals (smaller repre­sentations of the front panel controls and indicators) that pass data to and from the front panel. You connect these terminals using the wiring tool to pass data from one block to the next. The diagram may have multiple data paths and thereby sim­ultaneous operations. The LabVIEW system also has functional blocks to perform simple arithmetic functions, advanced Creating the front panel In LabVIEW, you first create the front panel to define the input and output parameters of the program. LabVIEW has controls and indicators (knobs, sliders, switches, LEDs, text boxes, charts and graphs) in hierarchical menus. Once an indicator or control is selected and placed on the panel, it can be moved, sized, labelled and configured in terms of data type, dimension, range, default and mechanical action. The user can import pic­ tures and controls to tailor a panel to a specific application. Fig.3: LabVIEW is a graphical programming system for developing data acquisition & instrument control applications on Macintosh computers, Windows PCs & Sun SPARCstations. February 1994  23 distribute VIs to users who can load and run VIs but cannot edit them or display their diagrams. This protects the propriety rights and integrity of VIs. The LabVIEW Run-Time System can serve as a low-cost test station or as an efficient way to package and resell VIs. Input/output (I/O) Most applications require the use of hardware for data collection, so users should also consider what types of I/O their application will require –plug-in data acquisition and/or in­ strument control. The software should work with a variety of hardware – it is then easy to integrate different types of hard­ware into one system. The data acquisition (DAQ) hardware should have ready-to-use instrument drivers available and it should be easy to add new drivers. LabVIEW has drivers for more than 300 GPIB, VXI and RS-232 instruments. The drivers consist of high-level functions with a front panel to operate each instrument. More importantly, each icon can be incorporated into a block diagram with other driver icons to build a complete test system. Data analysis Fig.4: Lubrizol Corporation in Wickliffe, Ohio (USA) uses a Macintosh Computer running LabVIEW in its high temperature fluid durability cycling tests. Lubrizol uses LabVIEW to create unique screens to easily acquire, analyse and save raw data from the tests. This process has eliminated many of the variables involved in the analysis of the data & streamlined the report generation process. acquisition and analysis routines and file I/O and network operations that store or re­trieve data in ASCII, binary or spread­sheet formats. LabVIEW also contains a formula node for typing in simple arithmetic equa­ t ions. For more complicated routines, the Code Interface Node (CIN) links external code to the diagram. This feature is import­ant for users that have already developed routines, like analysis algorithms, in a conventional language. LabVIEW includes extensive tools to develop, test and debug a VI system. The Help window describes each VI and its connec­ tions. The program immediately indicates incorrect wire connec­ tions with a dashed line. In addition, the Error window lists syntax errors. Execution highlighting traces the data paths during VI execution. The single-step mode and breakpoints 24  Silicon Chip also aid in VI debugging. LabVIEW has programming structures such as for loops, while loops and case statements for sequential, repetitive and branch­ing operations that determine if or how many times a set of functions will be executed. Graphical compiler LabVIEW is the only software of its type that features a graphical compiler – a system that compiles its block diagrams into machine code. This produces programs that execute at speeds comparable to compiled C programs. Consequently, LabVIEW programs execute 10 to 1000 times faster than those of any other graphi­cal instrumentation programming system. The graphical compiler also creates VIs for the LabVIEW Run-Time System. With this compact, low-cost version of LabVIEW, system developers can Users need to convert acquired data into meaningful re­sults. The Analysis VI libraries offer digital signal processing (DSP), digital filtering, statistics and numerical analysis functions. Also included are functions for array manipulation, complex arithmetic and statistical functions, Fast Fourier Trans­­ form (FFT) and Fast Hartley Transform (FHT) integration, differ­ entiation, convolution and correlation, power spectrum and pulse parameters; finite impulse response and infinite impulse response digital filters; win­ dowing functions; signal generation; linear, exponential and polynomial curve fitting; advanced statistics; and complex and matrix operations. As you can see, LabVIEW is a comprehensive solution to virtual instrument programming. It is intuitive and the resultant compiled programs run very fast. For further information on instrumentation programming and other data acquisition products, contact Tony O’Donnell, National Instruments Australia Corporation, PO Box 466, Ringwood, Vic 3134. Phone (03) 879 SC 9422 or fax (03) 879 9179.