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Sunday, August 17, 2008

Basics of Virtual Instrumentation

Basics of Virtual Instrumentation

LabVIEW programs are called virtual instruments, are VIs, because their appearance and operation imitate physical instruments, such as oscilloscopes and multimeters. Every VI uses functions that manipulate input from the user interface or other sources and displays that information or moves it to other files or other computers.

A VI contains the following three components,
• Front panel
• Block diagram
• Icon and connector pane

The Front panel serves as the user interface. The Icon and the connector pane identifies the interface to the VI so that one VI can be used in another VI. In general, a VI within another VI is called a sub VI. A sub VI corresponds to a subroutine in text based programming languages. The Block diagram contains the graphical code of VI used.
When a LabVIEW application is created, it is started at the top-level VI and inputs and outputs for the application are defined. Then a sub VI is constructed to perform the smaller tasks within the top-level VI. This modular approach is one of the strengths of LabVIEW. Hence it is possible to create complicated applications that are hierarchal in nature and reuse common elements within an application. The use of sub VI makes application easy to understand, debug and maintain.
The basic LabVIEW environment elements are the menus at the top of the front panel and block diagram windows, toolbar and free floating palettes like tools palette and controls palette.
The Tools Palette is available on the front panel and block diagram. A tool is a special operating mode of the mouse cursor. The cursor corresponds to the icon of the tool selected in the palette. The tools are used to operate and modify the front panel and block diagram objects. If automatic tool selection is enabled and the cursor is moved over the objects on the front panel or block diagram, LabVIEW automatically selects the corresponding tools from the tools palette. The tools palette can be also be used to modify the contents of the front panel or the block diagram. Each icon on the tools palette changes the behaviour of the cursor on LabVIEW so that operations like positioning, operating and editing tasks on VIs can be performed.

A Front Panel is built by placing controls and indicators from the controls palette. Each palette icon represents a sub palette, which contains control. A control is a front panel object that the user manipulates to interact with the VI. Simple examples of controls, buttons, slides, dials and text boxes. An indicator is also a front panel object that displays the data to the user.

Examples of indicators are graphs, thermometers, gauges. When control or indicator is placed in front panel, the corresponding terminal is automatically created in block diagram.


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