20120501 - introduction to dataflow

7/28/2019 20120501 - Introduction to Dataflow

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Gruppo di Misure Meccaniche e TermicheUNIBS - DIMI

A. Virtual Instruments (VIs)

The appearance and operation of VIs imitate physical instruments,

such as oscilloscopes and digital multimeters.

Virtual Instrument (VI) – A LabVIEWprogram




B. Parts of a VI

LabVIEW VIs contain three main components:

1. Front Panel 2. Block Diagram 3. Icon/Connector Pane




B. Parts of a VI – Front Panel

You build the front panel

with controls (inputs) and

indicators (outputs)

Front Panel – User interface for the VI




B. Parts of a VI – Block Diagram

Front panel objects

appear as terminals on

the block diagram

Block Diagram – Contains thegraphical source code




B. Parts of a VI – Icon/Connector Pane

• Icon: graphical representation of a VI

• Connector Pane: map of the inputs and outputs of a VI

• Icons and connector panes are necessary to use a VI as

a subVI− A subVI is a VI that is inside of another VI

− Similar to a function in a text-based programming

language

Icon Connector Pane




E. Front Panel – Controls Palette

• Contains the controls and indicators you use to

create the front panel

• Access from the front panel by selecting

View»Controls Palette




E. Front Panel – Controls & Indicators

• Controls

− Knobs, push buttons, dials, and other input devices

− Simulate instrument input devices and supply data to the block

diagram of the VI• Indicators

− Graphs, LEDs, and other displays

− Simulate instrument output devices and display data the block

diagram acquires or generates




E. Front Panel – Numeric Controls/Indicators

The numeric data type can represent numbers of various

types, such as integer or real

Numeric Indicator

Numeric Control

Increment/Decrement Buttons




E. Front Panel – Boolean Controls/Indicators

• The Boolean data type represents data that only has two

parts, such as True and False or On and Off

• Use Boolean controls and indicators to enter and display

Boolean (True or False) values• Boolean objects simulate switches, push buttons, and LEDs

BooleanControl

BooleanIndicator




E. Front Panel – Strings

• The string data type is a sequence of ASCII characters

• Use string controls to receive text from the user such as a

password or user name

• Use string indicators to display text to the user




E. Front Panel – Shortcut Menus

• All LabVIEW objects have

associated shortcut menus

• As you create a VI, use the

shortcut menu items to change

the look or behavior of frontpanel and block diagram

objects

• To access the shortcut menu,

right-click the object




E. Front Panel – Property Dialog Box

• Right-click a front panel

object and select

Properties to display

• The options available on

the property dialog boxare similar to the options

available on the shortcut

menu for that object



Gruppo di Misure Meccaniche e Termiche

UNIBS - DIMI

E. Front Panel – Configure Multiple Objects

• Select multiple objects to simultaneously configure shared

properties




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Configuring Your LabVIEW Environment

• Functions Palette

− Tack the Functions palette and select Customize»Change

Visible Categories then click Select All

• Controls Palette− Tack the Controls palette and select Customize»Change

Visible Categories then click Select All




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Pratice

Replicate the following front panel




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F. Block Diagram

Block diagram objects include the

following:

• Terminals

• SubVIs• Functions

• Constants

•Structures

• Wires




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F. Block Diagram – Terminals

• Terminals are:

− Block diagram appearance of front panel objects

− Entry and exit ports that exchange information between the

front panel and block diagram− Analogous to parameters and constants in text-based

programming languages

• Change the view type of a terminal by toggling the View as

Icon selection from the context menu




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F. Block Diagram Terminals




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B. LabVIEW Data Types – Terminals

Terminals visually communicate information about the data

type represented




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F. Block Diagram – Wires

• Transfer data between block diagram objects through wires

• Wires are different colors, styles, and thicknesses,

depending on their data types

• A broken wire appears as a dashedblack line with a red X in the middle

Scalar

1D Array

2D Array

DBL Numeric Integer Numeric String




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F. Block Diagram – Nodes

• Objects on the block diagram that have inputs and/or

outputs and perform operations when a VI runs

• Analogous to statements, operators, functions, and

subroutines in text-based programming languages• Nodes can be functions, subVIs, or structures

Nodes




UNIBS - DIMI

H. Selecting A Tool

• Create, modify, and debug VIs

using the tools provided by LabVIEW

• A tool is a special operating mode of the

mouse cursor • The operating mode of the cursor corresponds to the icon of

the tool selected

•

When using the Automatic Tool Selection, LabVIEWchooses which tool to select based on the current location of

the mouse




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Practice

Look and the block diagram of the previous practice and addthe following nodes:

• Add

•Multiply

•Random number •Greater than

•Less than

•Get date/time in seconds

Then use wires to connect all the elements of the blockdiagram leaving no terminal unconnected.

Target:

Simulate a given temperature value,

add a random noise of given amplitude,

then light up an alert when such atemperature is greater than setpoint and

another if is less than setpoint .




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Practice solution




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F. Block Diagram – Function Nodes

• Fundamental operating elements of LabVIEW

• Do not have front panels or block diagrams, but do have

connector panes

• Double-clicking a function only selects the function – doesnot open it like a VI

• Has a pale yellow background on its icon




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F. Block Diagram – SubVI Nodes

• SubVI: VIs that you build to use inside of another VI

• Any VI has the potential to be used as a subVI

• When you double-click a subVI on the block diagram, you

can view the front panel and block diagram of the subVI− The upper right corner of the front panel and block diagram

displays the icon for the current VI

− This is the icon that appears when you place the VI on a blockdiagram as a subVI




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F. Block Diagram – Wires

• Transfer data between block diagram objects through wires

• Wires are different colors, styles, and thicknesses,

depending on their data types

• A broken wire appears as a dashedblack line with a red X in the middle

Scalar

1D Array

2D Array

DBL Numeric Integer Numeric String




UNIBS - DIMI

B. LabVIEW Data Types – Terminals

Terminals visually communicate information about the data

type represented




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B. LabVIEW Data Types – Numerics

• The numeric data type represents

numbers of various types

• To change the representation

of a numeric, right-click thecontrol, indicator, or constant,

and select Representation

from the shortcut menu




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B. LabVIEW Data Types – Boolean

• Behavior of Boolean controls is specified by the mechanical

action

• In LabVIEW, the Boolean

data type is representedwith the color green




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Mechanical Action of Booleans

Use the Mechanical Action of Booleans VI located in the NI

Example Finder to learn about the different switch and latch

actions.




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B. Data Types – String

• A sequence of displayable or non-displayable ASCII characters

• On the front panel, strings appear as tables,text entry boxes, and labels

• Change the display type from the short-cutmenu: Normal, ‘\’ Codes, Password and Hex

• Edit and manipulate strings with the String functions on the

block diagram• In LabVIEW, the string data type is represented with the

color pink




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B. Data Types – Enum

An enum

represents a

pair of values,

a string and anumeric, where the

enum can be one

of a defined list of

values




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B. Data Types – Enum

• Enum: enumerated

control, constant, or

indicator

• Enums are useful because it is easier to manipulate numbers than strings

on the block diagram




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B. Data Types – Dynamic

• Stores the information generated or acquired by an Express

VI

• Non-Express VIs do not accept the dynamic data type

− To use a built-in VI or function to analyze or process thedynamic data type, you must convert the data type

− Numeric, waveform, or Boolean data indicators or inputs

automatically convert the dynamic data type when wired

• In LabVIEW, the dynamic data type is represented with the

color dark blue




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F. Block Diagram – Wiring Tips

• Press <Ctrl>-B to delete all broken wires

• Right-click and select Clean Up Wire to reroute the wire




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F. Block Diagram – Wiring Tips

Use the Clean Up Diagram tool to reroute multiple wires and

objects to improve readability

1. Select a section of your block diagram

2. Click the Clean Up Diagram button on the block diagramtoolbar




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I. Dataflow

LabVIEW follows a dataflow model for running VIs

• A node executes only when data are available at all of its

input terminals

• A node supplies data to the output terminals only when thenode finishes execution




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I. Dataflow – Quiz

Which node executes first?

a) Add

b) Subtract

c) Random Number d) Divide

e) Sine




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I. Dataflow – Quiz Answers

NO CORRECT ANSWER

Which node executes first?

a) Add –

possibly b) Subtract – definitely not

c) Random Number – possibly

d) Divide – possibly

e) Sine –

definitely not




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Summary—Quiz

1. Which function executes first: Add or Subtract?

a) Add

b) Subtract

c) Unknown




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Summary—Quiz Answer

1. Which function executes first: Add or Subtract?

a) Add

b) Subtract

c) Unknown




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I. Case Structures

•

Have two or more subdiagrams or cases• Execute and displays only one case at a time

• An input value determines which subdiagram to execute

•

Similar tocase

statements or if...then...else

statements in text-based programming languages




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I. Case Structures

• Case Selector Label: contains the name of the current caseand decrement and increment buttons on each side

• Selector Terminal: Wire an input value, or selector, todetermine which case executes




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Practice

Add to the previous practice a button in order to update the

string label ONLY if the button is pressed. Then run.

Change its mechanical action and run again.

Create a enum control with items “Heating”, “Cooling”,“Conditioning” and use its values to turn on label “too hot”

only, label “too cold” only, or both, according to the case

selected.

Make the string indicator display “Hi there!” if the user inputs

“Hello”, or the same input string if otherwise.




UNIBS - DIMI

I. Case Structures – Default Case

• You can specify a default casefor the Case structure

− If you specified cases for 1, 2,and 3, but you get an input of

4, the Case structure executesthe default case

• Right-click the Case structureborder to add, duplicate,

remove, or rearrange casesand to select a default case




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I. Case Structures – Input & Output Tunnels

You can create multiple input and output tunnels

• Inputs are available to all cases if needed

• You must define each output tunnel for each case




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I. Case Structures – Use Default if Unwired

Default values are:

Avoid using the Use Default If Unwired option on Casestructure tunnels

• Adds a level of complexity to your code

• Complicates debugging your code

Data Type Default Value

Numeric 0

Boolean FALSE

String Empty




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I. Case Structures – Boolean

Boolean input creates two cases: True and False




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I. Case Structures – Integer

• Add a case for each integer as necessary

• Integers without a defined case use the default case




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I. Case Structures – String

• Add a case for each string as necessary

• Strings without a defined case use the default case




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I. Case Structures – Enum

• Gives users a list of items from which to select

• The case selector displays a case for each item in the

enumerated type control




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I. Case Structures - Error Checking and Error

Handling

Use Case Structures inside VIs to execute the code if thereis no error and skip the code if there is an error




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D. While Loops

LabVIEW While Loop Flowchart Pseudo Code

Repeat (code);

Until Condition met;End;




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D. While Loops

• Iteration terminal: returns number of times loop has

executed; zero indexed

• Conditional terminal: defines when the loop stops

Iteration Terminal Conditional Terminal




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Practice

Add a STOP button to the previous practice, then includeeverything in a while loop in order to stop the program when theuser hit STOP.

Add a CHART and display the current iteration number on it.

Please note: from now on the RUN CONTINOUSLY buttonwon’t be used, use the RUN button instead.

AVOID HITTING THE ABORT BUTTON AT ALL COSTS!!




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D. While Loops – Tunnels

• Tunnels transfer data into and out of structures

• The tunnel adopts the color of the data type wired to thetunnel

•

Data pass out of a loop after the loop terminates• When a tunnel

passes data intoa loop, the loop

executes onlyafter data arriveat the tunnel




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Practice

Please open again the previous vi.

Using an input tunnel display the time the vi has been active

(in seconds) and stop it when such time is MORE THAN 20s

or if the user hits the STOP button.

Think about this: what are the differences between a simplewire and a tunnel? Can a wire HOLD data? Can a tunnel?




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F. Timing a VI

Why do you need timing in a VI?

• Control the frequency at which a loop executes

• Provide the processor with time to complete other tasks,

such as processing the user interface




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F. Timing a VI – Wait Functions

• A wait function inside a loop allows the VI to sleep for a set

amount of time

• Allows the processor to address other tasks during the wait

time• Uses the operating system millisecond clock




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F. Timing a VI – Elapsed Time Express VI

• Determines how much time elapses after some point in your

VI

• Keep track of time while the VI continues to execute

• Does not provide the processor withtime to complete other tasks




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Practice

Open task manager and run the previous vi.

Add to the previous practice the proper wait function in order

to force the loop time to 200ms

Open task manager and run the vi again.

Add to the previous practice the proper function in order todisplay a string version of time elapsed




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E. For Loops

LabVIEW For Loop Flowchart Pseudo Code

N=100;

i=0;

Until i=N:

Repeat (code;i=i+1);

End;




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E. For Loops

• Create a For Loop the same way you create a While Loop

• If you need to replace an existing While Loop with a For

Loop, right-click the border of the While Loop, and select

Replace with For Loop from the shortcut menu• The value in the count terminal (an input terminal)

indicates how many times to repeat the subdiagram




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E. For Loops – Conditional Terminal

You can add a conditional terminal to configure a For Loop to

stop when a Boolean condition or an error occurs




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E. For Loops – Numeric Conversion

• The number of iterations a For Loop executes must be specified innonnegative integers

• If you wire a double-precision, floating-point numeric value to the

count terminal, LabVIEW converts the larger numeric value to a 32-bit

signed integer




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• Normally, when you wire different representation types to theinputs of a function, the function returns an output in the larger

or wider format

• LabVIEW chooses the

representation that

uses more bits

• However, the For Loop

count terminal alwayscoerces to a 32-bit signed

integer


Coercion Dot




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• Avoid coercion for better performance

− Choose matching data type

− Programmatically convert to the matching data type




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Practice

Reproduce the following block diagram

What will be the result displayed in A? in B? in C? When?What will be the content of D?




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E. For Loops – Conditional Terminal

For Loops configured for a conditional exit have:

• Red glyph next to the count terminal

• Conditional terminal in the lower right corner




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E. For Loop/While Loop Comparison

For Loop

• Executes a set number of times unless

a conditional terminal is added

• Can execute zero times

• Tunnels automatically output an arrayof data

While Loop

• Stops executing only if the value at the

conditional terminal meets the

condition

• Must execute at least once• Tunnels automatically output the last

value




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A. Arrays

• An array consists of elements and dimensions

− Elements: data that make up the array

− Dimension: the length, height, or depth of an array

− An array can have one or more dimensionsand as many as (231) –1 elements per

dimension, memory permitting

• Consider using arrays when you work with

a collection of similar data and when youperform repetitive computations




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A. Arrays

• The first element shown in the array (3.00) is at index 1 andthe second element (1.00) is at index 2

• The element at index 0 is not shown in this image, because

element 1 is selected in the index display• The element selected in the index display always refers to

the element shown in the upper left corner of the element

display




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A. Arrays – Creating

1. Place an array shell on the front panel

2. Drag a data object or element into the array shell




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A. Arrays – 2D Array

• Stores elements in a grid

• Requires a column index and a row index to locate an

element, both of which are zero-based

• To create a multidimensional array on the front panel, right-click the index display and select Add Dimension from the

shortcut menu

• You also can resize the index display until you have as

many dimensions as you want




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A. Arrays – Initializing

• You can initialize an array, or leave it uninitialized

• For initialized arrays, you define the number of elements in

each dimension, and the contents of each element

• Uninitialized arrays have dimension but no elements




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A. Arrays – Creating Constants

• To create an array constant:

− Select an array constant on the

Functions palette

− Place the array shell on the block diagram− Place a constant in the array shell

• You can use an array constant to store

constant data or as a basis for comparison

with another array

• Array constants also are useful for passing data into a subVI




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A. Arrays – Auto-indexing

• If you wire an array to or from a For Loop or While Loop, youcan link each iteration of the loop to an element in that array

by enabling auto-indexing on tunnel

•

The tunnel changes from a solid square to the image shownabove to indicate auto-indexing




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A. Arrays – Auto-indexing Input

For Loop executesa number of times

equal to the number

of elements in the

array




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A. Arrays – Auto-indexing Input

If the iteration count terminal is wired and arrays of differentsizes are wired to auto-indexed tunnels, the actual number of

iterations becomes the smallest of the choices.




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Practice

Create a new VI having the following block diagram

Which values will be displayed by A,B and C?

O




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A. Arrays – Auto-indexing Output

• When you auto-index an array output tunnel, the outputarray receives a new element from every iteration of the

loop

•

Auto-indexed output arrays are always equal in size to thenumber of iterations

A A C i 2D A




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A. Arrays – Creating 2D Arrays

You can use two For Loops, one inside the other, to create a2D array

P ti



Practice

Open the thermostat practice VI,

Instead of generating a single temperature value per cycle,

create a set of 100 samples, put them in an array.

Display all the 100 samples on both the CHART and theGRAPH, while controlling the boolean indicators only on the

AVERAGE of the array

Please try to keep you block diagram as tidy as possible

20120501 - introduction to dataflow

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