LabVIEW for Physicists

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LabVIEW for Physicists
Ben Jeffery
24th October 2002
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What is it?

• Laboratory Virtual Instrument Engineering
  Workbench.

• Development environment for the G language.

• Produced by National Instruments for all major
  operating systems.

• Originally developed for communication with lab
  devices.

• A really useful tool.

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Why use LabVIEW?

• Easy to Learn - Intuitive.

• Easy to Use.

• No Syntax.

• Source code reads like schematic.

• Speed of development.

• Ready made controls for many instruments.

• Support.

• Easy and instant debugging.

• Power Instant Parallel threads.

• No memory concerns. (Within Reason)

• As versatile as text-based languages.

• Fun. Honest!

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Disadvantages

• Cost 1000 minimum.

• Hard to master advanced techniques.

• Code can become an impenetrable mess if youre
  not careful!

• Creating highly customized applications can
  become time consuming.

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Overview

• The Language
• Building an application
• Front Panel
• Source Diagram
• Programming
• Structures
• Built-in Functions
• User Vis
• Tips

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The G Language

• Completely graphical programming language.

• Source code is a diagram of nodes and wires.

• Data travels along wires.

• Flow of data controls execution. Not flow of code!

• Position on diagram is irrelevant.

• Being an expert in C can be a disadvantage

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A New Way of Thinking
Dataflow
NOT
Codeflow
UnlockCAL(HeaterMat) GetTempCAL(HeaterMat) Set
TempCAL(HeaterMat) LockCAL(HeaterMat)
Guiding Principle A node does not run until data
sits at all its wired inputs. Data
Dependency
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Building an Application

• Applications are called virtual instruments (VIs)

• Two parts to make

• Front Panel
• Source Diagram

• Always start with the front panel.

• Draw a pencil block diagram of the main functions
  of the diagram.

• Make this diagram in G code.

• Done.

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The Front Panel

• How the user interacts with G.

• Contains both controls and indicators.

• Each item on the front panel has a corresponding
  node on the source diagram.

• Many ready made controls means quick and easy
  construction of simple front panels.

• There is scope for customisation of the controls
  but this is mainly cosmetic.

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Controls and Indicators

• Front panel objects are either controls or
  indicators.

• The corresponding nodes are identified as shown.

• Controls are sources for data

• Indicators are sinks for data

Controls

• Constants are sources for data that do not
  appear on the front panel

Indicator
Constant
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Making the Front Panel
The best way to show this is in LabVIEW
Tips

• Use tab controls instead of cluttering the panel.
• Set limits on data entry to save programmatic
  checking.
• Keep to a consistent style.
• Dont go wild with the colours.
• Add text boxes when explanation is needed.
• Use schematic representation if a physical system
  is involved.
• Remember someone else might need to understand it
  in your absence!

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Building the diagram

• Double clicking any control on the front panel
  will take you to its node on the source diagram
  and vice-versa.
• Clicking window, show diagram (Ctrl-E) will
  also display the source diagram.

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Building the diagram

• Nodes are placed in a similar way to controls on
  the front panel.
• An extra tool is used on the source diagram

The Wiring Tool

• The most used, and most infuriating tool in
  LabVIEW.
• Avoid crossing wires.
• Click to join nodes together

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Types of Node
Types of nodes

• Indicators and Controls
• Functions
• Built in Functions
• User VIs (sub-routines)
• Structures

All have one or more terminals that usually only
accept one data type.
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Types of Wire
As terminals accept only defined data types, so
wires have defined types. This is indicated by
colour and style.

• Usual data types
• 8,16,32 bit Integers (Signed and Unsigned)
• Single, Double and Extended floating point
• Complex
• Boolean
• String
• Arrays
• Clusters (Combination of any above)

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Program Control
Run Once Program executes until all data is
sunk.
Run Continuously Run Once occurs
repeatedly.
Abort Execution Immediate halt.
Pause Execution Useful for debugging.
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Lets see some LabVIEW

• Well start with a simple example to get a feel
  for programming in LabVIEW

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A Closer Look

• Weve covered most of the basics of the
  environment. Now lets look at methods and
  techniques of programming.
• Structures
• Built In Functions
• User Vis
• Clusters
• Programming Tips

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Structures
How you control data flow

• Case Structure
• While Loop (with shift registers)
• For Loop
• Sequence
• Formula Node
• Events Structure

Understanding the operation of these is vital.
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Case

• Similar to the Case statement in C
• Contains one or more frames.
• The frame that executes depends on what is wired
  to the selection terminal.
• Many data types can be wired to the selector.

• Lets see it in LabVIEW

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While

• Similar to the While statement in C
• Contains one frame that is repeated
• Test for continuance is performed after execution
  so frame always executes once.
• i terminal gives number of current iteration
  (first frame is 0)
• stop terminal can be continue if true or
  stop if true selected from context menu.

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Shift Registers

• Selected via right-clicking the frame.
• Enables the result of an iteration to be passed
  to the next iteration.
• Can be used for any data type
• The results of older iterations can be accessed
  by making the left terminal larger.
• The initial value is set by wiring to the left
  terminal and the final iterations value is output
  at the right terminal.
• Results are kept between runs of a vi.

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For Loop

• Similar to the while loop but is executed a set
  number of times (wired to the N terminal)
• If 0 is wired to the terminal the frame does not
  run, do outputs will be invalid.
• Input arrays can be indexed automatically and if
  N is not wired the number of iterations will be
  the size of the largest input array.
• Shift registers can also be used here.

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Sequence

• Used to order events where no data dependency
  exists.
• Can have more than one frame but this should
  ALWAYS be avoided. (Like goto in C)

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Formula Node

• Used to avoid large numbers of arithmetic
  functions
• Has one or more inputs and outputs
• Uses C type syntax

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Event Structure

• Handles windows events
• Used for customisation of user interface
• Allows very fancy tricks!
• LabVIEW 6.1 only

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Built in Functions
Over 100 built in functions
Use the context help window to find what you need.
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Built in Functions

• Low Level
• Boolean Logic
• Arithmetic
• Comparisons
• These functions accept different data types and
  often arrays can be directly wired.

• Mid Level
• Array Manipulation (Transpose, interpolate,
  subset...)
• String Manipulation (Search, format, replace...)
• Time and Date

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Built in Functions

• High Level
• File I/O
• Device communication (GPIB, serial etc)
• Network communication (TCP, UDP, IrDA)
• Waveform manipulation (FFT, Filters, Analysis)
• Math
• Sound and Graphics
• Program Control
• Advanced Semaphores, Queues, Occurrences.

So check before you build your own
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User Vis
For often used routines or to prevent an excess
of code on your diagram create a user vi

• Create the vi as if it were a stand alone program
• Right click the icon to access the wiring and
  icon menu.
• Show connector and select a pattern of
  terminals and then use the wiring tool to select
  controls and indicators to be terminals.
• Edit the icon
• Place the vi where you need it using select a
  vi off the function pallet.

Lets see that in LabVIEW
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Clusters

• Collection of one or more items of data
• Best used as Named Clusters

• In this way you can carry related data in one
  wire and extract only the data you need when you
  need it.
• Also reduces the need for many terminals in a
  user vi.

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The Error Cluster

• LabVIEW standard, used in many built in VIs

Boolean Int String

• Passed along to each vi. If status is true the vi
  does not run as the preceding one gave an error.
• Lets illustrate this with an example

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Programming Tips

• Use a left to right layout
• Use named clusters for both neatness and ease of
  variable selection.
• If functions need to happen in a sequence
  establish a data dependency, so that the
  functions are connected in a chain.
• This is easily done using the error cluster.
• Remember that functions not connected and in the
  same frame will run in parallel.
• Make user VIs so that your code is modular.
• Experiment