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LabVIEW for Physicists

LabVIEW for Physicists

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LabVIEW for Physicists

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  1. LabVIEW for Physicists Ben Jeffery 24th October 2002

  2. 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.

  3. 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!

  4. Disadvantages: • Cost £1000 minimum. • Hard to master advanced techniques. • Code can become an impenetrable mess if you’re not careful! • Creating highly customized applications can become time consuming.

  5. Overview: • The Language • Building an application • Front Panel • Source Diagram • Programming • Structures • Built-in Functions • User Vi’s • Tips

  6. 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…

  7. A New Way of Thinking: Dataflow NOT Codeflow UnlockCAL(HeaterMat); GetTempCAL(HeaterMat); SetTempCAL(HeaterMat); LockCAL(HeaterMat); Guiding Principle: A node does not run until data sits at all its wired inputs. Data Dependency

  8. Building an Application: • Applications are called virtual instruments (VI’s) • 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.

  9. 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.

  10. 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

  11. 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. • Don’t 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!

  12. 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.

  13. 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:

  14. Types of Node: Types of nodes: • Indicators and Controls • Functions: • Built in Functions • User VI’s (sub-routines) • Structures All have one or more terminals that usually only accept one data type.

  15. 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)

  16. 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.

  17. Lets see some LabVIEW • We’ll start with a simple example to get a feel for programming in LabVIEW:

  18. A Closer Look • We’ve covered most of the basics of the environment. Now lets look at methods and techniques of programming. • Structures • Built In Functions • User Vi’s • Clusters • Programming Tips

  19. 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.

  20. 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:

  21. 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.

  22. 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.

  23. 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.

  24. 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)

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

  26. Event Structure: • Handles windows events • Used for customisation of user interface • Allows very fancy tricks! • LabVIEW 6.1 only

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

  28. 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

  29. 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

  30. User Vi’s: 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:

  31. 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.

  32. The Error Cluster: • LabVIEW standard, used in many built in VI’s 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:

  33. 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 VI’s so that your code is modular. • Experiment