Sistemi automatici di misura -...

Misure Elettroniche II Strumentazione virtuale

© 2006 Politecnico di Torino 1

Sistemi automatici di misura

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Bus standard per strumentazione

Strumentazione virtuale

Sistemi automatici di misura




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Conoscenza sistemisticafilosofia della strumentazione virtualelinguaggio di programmazione graficaintegrazione delle schede di acquisizione dati in un sistema di misura

Capacità di programmazioneprogrammare e gestire un sistema di misuraintegrare problematiche tipiche delle misureelettroniche in un sistema programmabileprogettare interfacce “friendly” per uno strumento di misura virtuale

Obiettivi della lezione



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Prerequisiti per la lezione

Elementi di programmazione:ambiente di programmazione ad oggetti

Fondamenti di Misure elettroniche:tecniche di campionamentoacquisizione e conversione A/D

Teoria dei segnali:teoremi del campionamento

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“Misure Elettroniche”U. PisaniEd. Politeko,Torino, 2000,

capitoli 6, 7, 10

Bibliografia per la lezione



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LabVIEW generalLabVIEW front panelLabVIEW block diagramWiring Objects on the Block DiagramControl structuresArrays and ClustersRunning the VI and debugging toolsCreating sub VIData acquisition Systems

Contenuti della lezione




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LabVIEW 1/2

LabVIEW stands for Laboratory Virtual Instrument Engineering Workbench

LabVIEW is a graphical programming languagethat uses icons instead of lines of text to create programs

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LabVIEW 2/2

LabVIEW programs are called Virtual Instruments, or VIs, because their appearance and operation imitate physical instruments

Every VI uses functions that manipulate inputfrom the user interface or other sources and displays that information or moves it to other files or other computers



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LabVIEW dialog box

Click the New buttonto create a new VI

Click the Open button to display an existing VI

Click the Configureto configure NI measurement and control tools (Data acquisition board)

Click Help for step by step instructions

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LabVIEW components 1/3

A VI contains the following three components:

front panel - Serves as the user interface

block diagram - Contains the graphical sourcecode that defines the functionality of the VI

icon and connector panel - Identifies the VI so that you can use the VI in another VI

a VI within another VI is called a subVI

a subVI corresponds to a subroutine in text-basedprogramming languages



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LabVIEW contains:

a comprehensive set of tools for acquiring, analyzing, processing, displaying, and storing data

tools to help you troubleshooting your code

LabVIEW allows to communicate with hardwaresuch as data acquisition, vision, and motioncontrol devices and GPIB, PXI, VXI, RS-232, and RS-485 instruments

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The front panel is the user interface of the VI

The code is contained in the block diagram

After you build the user interface, you add code using VIs and structures to control the front panel objects




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LabVIEW front panel 1/2

The user interface, or front panel, appears witha gray background and includes controls and indicators



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LabVIEW front panel 2/2

The title bar of the front panel indicates that thiswindow is the front panel for the Generate and Display VI

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Controls and Indicators 1/2

Controls are knobs, push buttons, dials, and other input devices

Indicators are graphs, LEDs, and other displays

Controls simulate instrument input devices and supply data to the block diagram of the VI



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Controls and Indicators 2/2

Indicators simulate instrument output devices and display data the block diagram acquires or generates

Controls and indicators are located on the Controls palette of the Window menu of the front panel

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Front panel Controls 1/3

You can configure the controls on the front panel to perform tasks depending on what you wantyour VI to do



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Some examples of controls:Numeric Controls

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Some examples of controls:Buttons & Switches



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Place a Control on the Front Panel 1/2

Click the icon Num Ctrls toaccess the NumericControlspalette

Select the proper control and place it on the front panel by drag and drop

DRAG AND DROP

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Place a Control on the Front Panel 2/2

Notice that when you move the cursor over iconson the Controls palette, the name of thatsubpalette appears in the grey space above allthe icons on the palette

When you idle the cursor over any icon on anypalette, the full name of the subpalette, control or indicator appears



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Front panel Indicators 1/3

Many physical instruments have output devicesto display numerical values, graphs, and LEDs

On the front panel you can configure properindicators like graphs, LEDs, and other displaysthat simulate the output devices on a physicalinstrument

The displayed data are acquired or generated bythe block diagram

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The Indicatorsare located in the Controls Palette

An example:Graph Indicator



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An example:LED Indicator

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Place an Indicator on the Front Panel

Apply same Controls procedure for placingIndicators

DRAG AND DROP



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Graphs and Charts 1/2

Use graphs and charts to display data in a graphical form

Graphs and charts differ in the way they display and update

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Graphs usually collect the data in an array

Plot the data to the graph, as a spreadsheet thatfirst stores the data then generates a plot

X

Y



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Chart appends new data points to those alreadyin the display to create a history in the time

Time running




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LabVIEW block diagram 1/2

From Window menu of the front panelSelect Show Block Diagram to display the block diagram

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LabVIEW block diagram 2/2

The block diagramappears with a whitebackground and includes VIs and structures that control the front panel objects

The title bar of the block diagram indicatesthat this window is the block diagram for the Generate and Display VI



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Front panel & Block diagram

The block diagram and the front panel can beboth visible

If the front panel isnot visible, you can display the front panel by selecting Window⇒ Show Front Panel

By pressing the <Ctrl-E> keys you can open the block diagram or the front panel

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Block Diagram objects

Block Diagram Terminals

Block Diagram Nodes

Functions



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Block Diagram Terminals 1/3

A terminal is any point to which youcan attach a wire, other than toanother wire

LabVIEW has control and indicatorterminals, node terminals, constants, and specialized terminals on structures

You can configure the terminals on the block diagram to appear as icon ordata type terminals by right clikingon the icon and selecting View as Icon

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Control terminals have a thicker borderthan indicator terminals

Arrows appear on front panel terminalsto indicate whether the terminal is a control or an indicator

Output arrow for control

Input arrow for indicator



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The terminals represent the data typeof the control or indicator

For example DBL stands for doubleprecision floating point numeric data

The color and symbol of each terminal indicate the data type of the control or indicator

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Data terminal codes

Examples of Control and Indicator Terminal Code



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Block Diagram Nodes

Nodes are objects on the block diagram thathave inputs and/or outputs and performoperations when a VI runs

They are analogous to statements, operators, functions, and subroutines in text-basedprogramming languages




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Writing program by wiring objects 1/5

The program code is realized by connecting the objects positioned on the block diagramEach object presents one or more input and output terminalsTerminals are entry and exit ports that exchange information between the front panel and block diagramMove the cursor over the arrowof the object terminal, notice how the cursor becomes a wire spool, (the Wiring tool )

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Use the Wiring tool to wire objects together on the block diagram



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Each wire has a single data source, but youcan wire it to many VIs and functions

Wires are different colors, styles, and thicknessesdepending on their data types

A wire segment is a single horizontal or verticalpiece of wire

LEGENDA

1 Segment

2 Bend

3 Junction

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A bend in a wire is wheretwo segments join

The point at which two or more wire segments joinis a junction

LEGENDA

1 Segment

2 Bend

3 Junction



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You must wire all required block diagramterminals otherwise, the VI is broken and will notrun

A broken wire appears as a dashed black line with a red X in the middle

Broken wires occur for a variety of reasons, such as when you try to wire two objects withincompatible data types

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Block Diagram Nodes

LabVIEW includes the following types of nodes:Functions - Built-in execution elements, comparable to an operator, function, or statementSubVIs - VIs used on the block diagram of another VI, comparable to subroutinesStructures - Process control elementsFormula Nodes - Resizable structures for entering equations directly into a block diagramExpression Nodes - Structures for calculating expressions, or equations, that contain a single variable…



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Block Diagram Function palette

From Block DiagramWindow menu select Show FunctionPalette that contains VIs, functions, block diagram structures, and constants

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Function palette 1/4

The icons of the Function palette allow to:

Input generate signals, generate drivers for I/O and instruments...Analyze signals (Spectral anaysis, filtering, statistics...)



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Execute and control software structuresArithmetic and Comparison tools

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Signal Manipulation (merge signals, split signals…)User libraries and other functions



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The Express VIs and structures located on the subpalettes of the Functions palette are a subset of the complete set of built-in VIs, functions, and structures available in LabVIEW

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An example of Input function 1/2

Simulate Signal:

type of signalssine wavesquare wavetriangle wavesawtooth wavenoise signal



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An example of Input function 2/2

setting options:

signal typefrequency (Hz)phase (deg) amplitudeoffsetduty cycle (%)add noisenoise type

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Simulate Signal Icon

INPUTPARAM

OUTPUTDATA



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Simulate Signal Properties 1/3

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Execution Control (Exec Ctrl) 1/2

While Looploop structure that repeats a section of code until a condition is met

Flat Sequence structureprogram control structure that executes its subdiagrams in numeric defined order

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Execution Control (Exec Ctrl) 2/2

Case structure

conditional branchingcontrol structure, that executes one of its subdiagramsbased on the input to the Case structure. It is the combination of the IF, THEN, ELSE, and CASE statements in control flow languages



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While loop

The default conditionto STOP the whileloop is clicking a stop button on the front panel

All the code includedin the loop rectangleis executed

While loop

Loop index

Condition

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Flat Sequence structure

The Flat Sequence structure displays all theframes at once and executes the frames from left to right until the last frame executes

Flatsequence



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Stacked Sequence structure

The stacked sequencesubstitutes in a more compact mode the flatsequenceUse it to save space in the block diagramTo scroll through the available subdiagrams, click the decrement and increment arrows in the structure selector label

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Case structure

Case selector terminal

Input tunnelOutput tunnel

CASE FALSE

CASETRUE



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All functions

Includes a miscellaneous of icons of functions, I/O handling, operators and so on

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All functions -> Structures

The palette Structures includes icons of alreadydescribed structures and other ones like

For Loop

Formula Nodes

Event Structures

...



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For Loop

Executes its subdiagramN times, where N is the value wired to the count(N) terminal

The iteration (i) terminal provides the current loop iteration count, which ranges from 0 to N -1

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Formula Node

Evaluates mathematicalformulae and expressionson the block diagram

The Formula Node syntax is similar to the syntax used in text-based programminglanguages

Remember to end assignments with a semicolon (;)



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Event Structure

The event structure contains one or moresubdiagrams that execute depending by an event

An event is an asynchronous notification thatsomething has occurred

Events can originate from:

the user interfaceexternal I/Oother parts of the program

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Event types

User interface events include:

mouse clickskey presses, and so on

External I/O events include hardware timers or triggers that signal when data acquisition completes or when an error condition occurs

Programmatically generated events can be usedto communicate and execute different parts of the program



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Shift registers 1/4

When, using For Loops or While Loops, you wantto transfer values from one loop iteration to the next, use shift registers

A shift register appears as a pair of terminals, directly opposite each other on the vertical sidesof the loop borders

LabVIEW transfers the data connected to the right sideof the register to the left sidein the next iteration

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Shift registers 2/4

Create a shift registerby right-clicking one of the vertical bordersof a loop and selectingAdd Shift Registerfrom the shortcut menu



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Shift registers 3/4

Initialize a shift register by wiring a control or constant to the shift register terminal on the leftside of the loop

Initialize

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Shift registers 4/4

The data you wire to the terminals of each shiftregister must be the same type

Initialize



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Strings

A string is a sequence (displayable or not) of ASCII characters, that provides a platform-independent format for information and data

On the front panel, strings appear as tables, textentry boxes, and labels

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Using the Strings

Some of the more common applications of strings include the following:

creating simple text messagespassing numeric data as character strings to instruments and then converting the strings to numericsstoring numeric data to disk. To store numerics in an ASCII file, you must first convert numerics to strings before writing the numerics to a disk fileinstructing or prompting the user with dialogboxes




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Arrays

Arrays is a group of data elements of the sametype

An array consists of elements and dimensions: elements are the data that make up the array a dimension is the length, height, or depth of an array

An array can have one or more dimensions (as many as 231 – 1 elements per dimension memory permitting)



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Grouping Data Arrays

You can build arrays of numeric, Boolean, path, string, waveform, and cluster data types control or indicator

Consider using arrays when you work with a collection of similar data and when you performrepetitive computations

Arrays are ideal for storing data you collect from waveforms or data generated in loops, where each iteration of a loop produces one element of the array

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Array functions

The Array functions allow to create and manipulate arrays, such as the following tasks:

extract individual data elements from an array

add individual data elements to an array

split arrays



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Example of 1D array 1/2

An example of an array is a waveformrepresented as a 1D numeric array in which eachsuccessive element is the voltage value at successive time intervals

NoteThe index is zero-based, which means it is in the range0 to n – 1, where n is the number of elements in the array

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A more complex example of an 1D array is a graph represented as an array of points whereeach point is a cluster containing a pair of numerics that represent the X and Y coordinates



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You can generalize the 1D waveform arrayexample to two dimensions by adding rows tothe array

Figure shows a collection of waveformsrepresented as a 2D array of numerics

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The row index selects the waveform, and the column index selects the point on the waveform



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Creating an array 1/2

An array control or indicator on the front panel is created by placing anarray shell on the front panel

Drag a data object or element into the array shell

The array shell automatically resizesto accommodate the new object

Indexdisplay

Array element

Array shell

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Creating an array 2/2

To create a multidimensional array right-clickthe index display and select Add Dimension fromthe shortcut menu

2D Array

LEGENDA

1 Row Index

2 Column Index

3 Value at row, column



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Clusters 1/3

Clusters are groups of data elements of mixedtypes

Clusters differ from arrays in that they are a fixed size

Like an array a cluster is either a control or anindicator

A cluster cannot contain a mixture of controlsand indicators

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Clusters 2/3

Bundling several data elements into clusters:

eliminates wire clutter on the block diagram reduces the number of connector pane terminalsthat subVIs need assign the cluster to a terminal on the connector pane

Cluster elements have a logical order unrelated to their position in the shell

The first object you place in the cluster is element 0, the second is element 1, and so on



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Clusters 3/3

If you delete an element, the order adjustsautomatically

The cluster order determines the order in whichthe elements appear as terminals on the Bundleand Unbundle functions on the block diagram

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Cluster example 1/2

Cluster of 3 control elements (example)



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Cluster example 2/2

Cluster output is reordered by unbundle operator in 3 corrispondent indicators (example)

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Build a Cluster 1/3

After creratingthe Clusterelements in the front panel

Open Arrays&Clusters palette

SelectCluster



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Build a Cluster 2/3

Drag and drop Cluster box in the front panelResize the cluster areaDrag and drop the cluster elements intothe clusterFollowing a defined order

Element 0

Element 1

Element2

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Build a Cluster 3/3

Cluster Icon will appearon the block diagram

Right-clicking the clustericon select the clusterpalette

Cluster functions palettewill appear

Cluster Icon



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Cluster functions

The Cluster functions allow to create and manipulate clusters, such as the following tasks:

extract individual data elements from a cluster

add individual data elements to a cluster

break a cluster out into its individual data elements




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Running a VI executes your solution

A Run button is present in the block diagramwindow as well in the front panel

Clicking the Run button executes the program

Clicking the Run Continuosly button the program executes repetitively

To indicate that the VI is running, the buttonschanges to a darkened arrow

Running the VI

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Block Diagram Data Flow 1/3

Text-based programming languages follow a control flow model (the execution order is thesequential order of the program code)

LabVIEW VIs run following a data flow model



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A block diagram node executes when all itsinputs are available

When a node completes execution, it supplies data to its output terminals

The output data pass to the next node in the data flow path

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In LabVIEW you can create block diagrams that have“simultaneous” operations

For example, you can runtwo While Loops“simultaneously” and display the results on the front panel



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Broken VIs

The Run button appears broken when the VI you are creating or editing contains errors

If it is still broken when you finish wiring the block diagram, the VI is broken and cannot run

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Execution Highlighting

Clicking the HighlLight Execution button , when running the program, the movement of flow data from one node to another is shownusing bubbles that move along the wires

Lamp lights

Data move along the wire



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Single-step through a VI to view each action of the VI on the block diagram as the VI runsThe single-stepping buttons affect execution onlyin a VI or subVI in single-step mode

Move the cursor over the Step Over, Step Into, or Step Out button to view a tip strip thatdescribes the next step if you click that button

Debug by Single-Stepping

Step Over Step OutStep Into

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Debugging by Probe 1/2

You can set a Probe in the block diagram tovisualize data passing through a wireA window numbered with the correspondingprobe number will show you the data



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Debugging by Probe 2/2

To set a probe when the VI is not running

Right-click a wire and select CustomProbe»Generic from the shortcut menu to use the generic probe

The generic probe displays the data

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Debugging by Probe tool 1/2

Use the Probe tool to check intermediate valueson a wire as a VI runsBy positioning the mouse on the wire the probe tool will appear

Probe tool



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Debugging by Probe tool 2/2

If data are available, by clicking the probe toolthe probe immediately updates while the VI isrunning

Probe tool

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Debug by Breakpoints

Use the Breakpoint tool to placea breakpoint on a VI, node, or wire on the block diagram andpause execution at that location

When you set a breakpoint on a wire, execution pauses after data pass through the wire

To set a breakpoint Right-click a wire and select Breakpoint fromthe shortcut menu



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Using Built-In VIs and Functions

LabVIEW includes VIs and functions to help youbuild specific applications, such as:

data acquisition VIs and functionsVIs that access other VIsVIs that communicate with other applications

LabVIEW includes hundreds of example VIs youcan use and incorporate into VIs that you create reducing development time

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Instrument Control

You can use the built-in VIs and functions tocontrol external instruments such asoscilloscopes, voltmeter frequency meters ...

To control instruments in LabVIEW, you musthave the correct hardware installed, powered on, and operating on your computer

The VIs and functions you use to control instruments depend on the instrumentationcommunication protocols your hardware supports



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Data acquisition 1/2

Use the Data Acquisition VIs and functions toacquire data from DAQ devices

To use these VIs, you must have the NI-DAQ driver software and DAQ hardware installed

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Data acquisition 2/2

After you acquire data, you can use the built-inAnalyze, Report Generation, and Mathematics VIs and functions to:

analyze data

generate reports

perform mathematical operations on that data



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Communicate with Other Applications 1/2

Use the File I/O VIs and functions to write data to or read data from other applications, such asMicrosoft Excel

You can use these VIs and functions to generate reports or incorporate data from anotherapplication in the VI

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Communicate with Other Applications 2/2

Use the Communication VIs and functions to transfer LabVIEW data across the Web using a communication protocol such as FTP and to buildclient-server applications using communication protocols




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Building SubVIs as Icons

After you build a VI and create its icon and connector pane, you can use it in another VI

A VI called from the block diagram of another VI is called a subVI

A subVI corresponds to a subroutine in text-based programming languages

A subVI node corresponds to a subroutine call in text-based programming languages



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To create a SubVI

Click the Select a VIicon on the Functions palette

Browse and select the VI, and place it on the block diagram to create a subVI

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Setting up the Connector Pane 1/2

To use a VI as a subVI, you need to build a connector pane

The connector pane is a set of terminals that corresponds to the controls and indicators of thatVI



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To define a connector pane, right-click the icon in the upper right corner of the front panel windowand select Show Connector from the shortcutmenu

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The connector pane replaces the icon

Each rectangle on the connector pane representsa terminal

Use the rectangles to assign inputs and outputs

INPUTS OUTPUTS




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Measurement data acquisition

Acquiring data include plug-in DAQ and instrument devices, GPIB instruments, PXI (PCI eXtensions for Instrumentation) instruments, and RS-232 instruments

A general-purpose DAQ device is a device that:

acquires analog or digital data also on multiple channelsgenerates analog and digital signals also on multiple channels



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On board data acquisition

Typically you connect these devices directly tothe internal bus of a computer through a plug-inslot (PCI slot of a desktop computer or the PCMCIA slots of a laptop computer)

DAQ device can be external and connects to the computer through an existing port (ex., such asthe USB connector, serial port or Ethernet port)

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External data acquisition

Many instruments are external to the computer and do not rely on a computer to take a measurement

That is the case of a GPIB instrument, connectedto the PC by a standard IEEE488 board

All the solutions are supported by LabView that can programatically control and monitor the instruments, collect and further process data and also present/store in files



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NI-DAQ 7.0

NI-DAQ 7.0 contains two NI-DAQ drivers eachwith its own Application Programming Interface (API), hardware configuration, and software configuration:

Traditional NI-DAQ NI-DAQmx

Traditional NI-DAQ is an upgrade to NI-DAQ 6.9.x, the earlier versionof NI-DAQ: has the sameVIs and functions and works the same way asNI-DAQ 6.9.x.

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NI-DAQmx

NI-DAQmx is the latest NI-DAQ driver with new VIs, functions, and development tools forcontrolling measurement devices

The advantages of NI-DAQmx over previousversions of NI-DAQ include the DAQ Assistant forconfiguring channels and measurement tasks fora device



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Instrument driver 1/3

An instrument driver is a collection of functionsthat control and operate the instrument thatsimplify instrument programming to high-levelcommands (not need to learn low-level, instrument-specific syntax)

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Using instrument drivers provides the following advantages:

quickly build complete systems

reduce software development costs

help make test programs easier to maintain(because instrument drivers contain all the I/O for an instrument within one library, separate from other code, which is easy to upgrade when or if the hardware changes)



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NI provides more than 2,200 instrument drivers from more than 150 vendors

Refer to the NI Instrument Driver Network at ni.com/idnet for a list of available instrument drivers

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Types of Data Acquisition boards

Easy I/O → Base I/O VIAnalog Input →Analog Output →Counter/Timer → Intermediate VIDigitalInput and Output →Calibration and Config →Advanced → Advanced VIDAQ Utilities → Utility DAQ



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Waveform Acquisition VI

Example of VI icon for multichannel sampling and acquisition

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Multiple channel Waveform Acquisition

Sottoprogramma che effettua la conversione tra la tensione misurata da un sensore e la

temperatura

Sottoprogramma che effettua la misura delle tensioni dei

sensori mediante la scheda di acquisizione dati

Controllo che calcola il valore medio di un vettore

Controllo che seleziona una colonna nella matrice con le

tensioni misurate



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Transducer signal acquisition

PhysicalPhenomena

Transducers SignalConditioning

DAQBoard

PersonalComputer

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Analog DAQ board Features

Choose PCI E Series DAQ Boards with• 20 kS/s to 5 MS/s• 12 or 16-bit resolution• 16 to 64 analog inputs• Multiplexed or simultaneous S/H• 8 to 32 digital I/O lines• Calibration certificates for NIST traceability

AO Features• Up to 32 analog outputs• Up to 1 MS/s update rates• Up to 16-bit resolution• 0-20 or 2-20 mA current sinks• 8 digital I/O lines• Up to two 24-bit counter/timers• Digital triggering



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Digital IO Board Features

DIO Features• Up to 76 Mb/s• Up to 96 I/O lines• Pattern generation• Handshaking• Pattern and change detection• Start/stop triggering• Electromechanical relays and optical isolation

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Multiple channel SCXI system

Per gentile concessione della National Instruments



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Digital PXI IO Board


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PXI System





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Si elencano alcuni argomenti che devono risultare chiari dallo studio della lezione:

tipologia degli strumenti utilizzabili per progettare un front panel ed un block diagramtipologia degli strumenti per presentare segnali e informazioni di misuratipologia degli strumenti per elaborare e processare segnalistrutture per il controllo per la programmazionestrumenti di debuggestione di schede di acquisizione dati

Approfondimenti



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Sommario della lezione

Strumentazione virtualeLabVIEW generalLabVIEW front panelLabVIEW block diagramWiring Objects on the Block DiagramControl structuresArrays and ClustersRunning the VI and debugging toolsCreating subVIData acquisition Systems

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