Intermediate LabVIEW Tutorial June 27, 2011

Ayyappa Vemulkar, HMC 2013

This tutorial assumes that the user has programmed in LabVIEW before. If you haven’t used

LabVIEW before, it is advised that you first go through the following references:

1. Getting Started Manual on the National Instruments website -

http://digital.ni.com/manuals.nsf/websearch/EC6EF8DE9CB98742862576F7006B0E1E

2. Introduction to NI LabVIEW - http://www.ni.com/gettingstarted/labviewbasics/

The tutorial has been tested for LabVIEW 2009 and 2010, and may be done in teams of two.

Objectives

By the end of the tutorial the student will:

1) Learn how to install, test and use a Data Acquisition Device (DAQ) using the

Measurement and Automation Explorer (MAX) software

2) Review basic programming concepts in LabVIEW such as understanding the LabVIEW

environment; creating Sub Vis; using functions, loops and structures; and displaying and

working with different data types

3) Learn how to communicate with and control instrumentation over RS-232 and GPIB

Before you begin:

Create a New Folder in your Charlie directory for your LabVIEW work. You will save all

your VIs to this folder so that LabVIEW can find them.

Saving your VI frequently is good, but know that once you choose to save your VI, it

resets the ‘undo call’ (Ctrl-Z) and you will not be able to go back on any steps you made

before saving your VI.

Think node-to-node programming and not line-by-line.

Avoid crossing wires while connecting different objects on the block diagram. Crossed

wires can be confusing to read and follow at times.

When in doubt, DEBUG! Use the light bulb icon on the status toolbar on the block

diagram to highlight the path of the flow of data.

Place output indicators at multiple steps during the data flow path of your VI to keep

track of the output as it changes through the VI.

PLAN PLAN and PLAN! Plan out the various stages of your VI/Program prior to placing

objects on the block diagram. This will make it easier to end up with a clean picture of

code and will also allow you to pick out the functions that best suit your needs.

PRACTICE! The only way to get better at LabVIEW is to practice.

How to use this Tutorial:

Each section of this tutorial is divided into three parts: The Objectives, Background Information,

and Exercise. Each section addresses a “big picture idea.” The Objectives aim at telling the

reader the important topics that will be addressed within the section. The Background

Information acts as a resource, defining terms and attributes of LabVIEW you will need to

understand prior to doing the exercise and to fully comprehend the big picture idea of the

section. If you are not a beginner, it is advised you skim through the Background Information

and only read about topics you are not comfortable with. The Exercise is designed to be concept-

based as opposed to testing your programming skills. Apart from simply following directions, it

is advised, particularly through the later exercises, that you try thinking through the problem

statement at hand and planning how you might use LabVIEW as a tool to solve the problem.

Introduction:

LabVIEW, short for Laboratory Virtual Instrumentation Engineering Workbench, is a visual-

based programming environment created by National Instruments. LabVIEW is an industry

standard used for data acquisition (through DAQ devices), instrument control, laboratory

measurements, and industrial automation.

Bill of Materials

The following materials shown in Table 1 are required for the tutorial. They can all be checked

out from the stockroom and should all be available during the proctored lab session.

Component Description Supplier Manufacturer Part # Unit Price Quantity Total

Resistor (available in the stockroom)

220 Ω resistor, ¼ W, through-hole DigiKey P220CATB-ND $0.095 1 $0.095

Resistor 2.2 kΩ resistor, ¼ W, through-hole DigiKey 2.2KQTR-ND $0.074 1 $0.074

Capacitor Cap, Tantalum, 10 nF Capacitor, through-hole DigiKey 478-1832-ND $0.52 1 $0.52

Protoboard Solder less Breadboard DigiKey 438-1046-ND $13.82 1 $13.82

LED Through-hole LED DigiKey C4SMF-BJS-

CR0U0452TB-ND $0.27 3 $0.27

Photodiode Infrared through-hole, Dome shaped DigiKey. 751-1045-2-ND $0.95 1 $0 .95

Speaker Available in the stock room N/A N/A N/A 1 N/A

Ni-myDAQ Data Acquisition Device National Instruments

N/A $200 1 $200

GPIB to USB Cable

National Instruments

N/A $495 1 $495

Serial to USB Cable

RS 232 Port adapter National Instruments

N/A $135 1 $135

Agilent 33250A

Waveform Generator Agilent Technologies

N/A N/A 1 N/A

Hp/Agilent 54600A

Oscilloscope, 2 channel, 100MHz

Agilent Technologies

N/A N/A 1 N/A

HP E3620A DC Power Supply, 35 W, Triple Output

Agilent Technologies

N/A N/A 1 N/A

Elenco LCM-1950

Digital Mutlimeter Elenco N/A N/A 1 N/A

Table 1– Bill of Materials

Section 1 – Understanding your DAQ Device

Objectives: By the end of this section the student will learn how to:

1) Use MAX to self-test a DAQ device

2) Determine the input/output ports on a DAQ device

Background Information

A Data Acquisition (DAQ) device is the “means by which physical signals, such as voltage,

current, pressure and temperature are converted into digital formats and brought into the

computer”1. This section describes three key concepts: DAQ resolution, input configuration, and

the Measurement and Automation Explorer (MAX).

Resolution of a DAQ device: The resolution is the smallest increment of a signal that

can be determined by the device. One may determine the resolution of a device from the

number of bits in the A/D or D/A (digital/analog) converter and the full-scale range of the

device. For example: the resolution of a 16 bit device with a full-scale range of 0 to 10 V

is 10/(216

) V = 153 μV. (Note that noise may cause the device to have an accuracy that is

less than the resolution.)

Input Configuration: LabVIEW supports three input configurations of the channels on

the DAQ, as shown in Figure 1:

1. Differential: Typically used to measure small voltage differences between two input

voltages, especially if there is a possibility of a large common mode voltage. You

need 3 connections: a high side, a low side and a path to ground. The path to ground

is important; if the inputs are purely floating, the voltages may drift outside the valid

voltage range, a problem that is solved by connecting one of the input voltages to

ground using a resistor (usually valued around 10 kΩ). A typical application of this

configuration is for a thermocouple.

Note: When using the myDAQ, the preferred analog input connection is differential.

2. Referenced Single-Ended (RSE): This configuration is used when all measurements

are referenced to a common ground, usually the ground pin on the DAQ. You need

two connections, the input signal and a path to ground. The most common application

of RSE is for a power supply, oscilloscope, or waveform generator.

1 Source: Labview Measurements Manual

3. Non-Referenced Single-Ended Inputs (NRSE): Used if you don’t want to connect the

ground of the circuit to the ground of the DAQ. This setup is not very common in

practice.

Figure 1– Different input configurations based on the input signal type. The image was

taken from NI.com – Field Wiring and Noise Considerations for Analog Signals,

[http://zone.ni.com/devzone/cda/tut/p/id/3344]

MAX (Measurement and Automation Explorer): MAX is the software interface

provided by National Instruments that allows you to communicate with and test all

National Instrument Devices, without programming.

Equipment required for this section:

1. Agilent 33250A Waveform Generator

2. DAQ Device [NI-myDAQ]

3. LED

4. Resistor (220 Ω)

Exercise 1

Problem statement – Self-test a myDAQ device and test the Analog Input channels and the

Digital Output channels.

1. Plug the myDAQ into a USB port. If Windows detects the new device and prompts you for an

action, choose to take no action.

2. Launch MAX by double-clicking the icon on the desktop or by selecting

Start » Programs » National Instruments » Measurement & Automation.

3. Expand the Devices and Interfaces section to view the installed National Instruments devices.

MAX displays the National Instruments hardware and software currently available to the

computer.

4. The device number appears in quotes following the device name. The data acquisitions VIs

use this device number to determine which device performs DAQ operations. For example, your

hardware may be listed as NI myDAQ, as shown in Figure 2. If you are using LabVIEW 2010

and are unable to see your device, Expand the NI-DAQmx Devices section to view the installed

hardware that is compatible with NI-DAQmx.

Figure 2– The MAX window after selecting a device

5. Perform a self-test on the device by right-clicking on it and choosing Self-Test.

6. Check the pinouts for your device. Right-click the device and select Device Pinouts or click

“Device Pinouts” along the top of the center window. MAX will display the function of each pin

on the DAQ device. For example, the myDAQ has eight digital input/output (DIO) pins, two

differential Analog Input (AI) pin pairs, two Analog Output (AO) pins, and 5V, +15V, -15V, and

GND pins. The myDAQ case is already well marked with all the pin information displayed by

MAX, but some devices are not marked as well.

7. Open the test panels. Right-click the device and select Test Panels. With test panels, you can

test the inputs and outputs without doing any programming.

8. To test the Analog Input in differential mode, we will use an Agilent waveform generator.

Switch on the waveform generator. Caution: the DAQ channels are configured to handle at

most ±10 V. Ensure that the output button on the signal generator is switched on. Generate a 100

Hz sine wave with a peak-to-peak amplitude of 1 V. Ensure that the waveform generator is set to

drive an unterminated cable, by pressing Utility » Output Setup » High Z.

Be sure there is a pair of minigrabber clips attached to a BNC cable connected to the output of

the waveform generator. Using a straight-slot screw driver, attach wires to the 0+ and 0

- Analog

Input channels. Connect the positive output clip to the 0+ wire and the ground clip to the 0

- wire,

as shown in Figure 3. If you are using a different DAQ device, you will need to determine the

appropriate pinout for that device.

Figure 3– The schematic showing the setup for the Analog Input test

On the Analog Input tab of the test panels, change Mode to “Continuous” and Rate to “10,000

Hz” and leave the Configuration as “Differential”. Click “Start” and verify that the graph

matches your expectations. Vary the signal generated and check the results. Click “Stop” when

you are done.

9. To test the Digital Output, we will use an LED. Connect one end of the 220 resistor to the

+5V channel and the other end to the anode (positive terminal) of the LED (the positive side is

the longer lead). Then connect the cathode (negative terminal) of the LED to the DIO channel 0,

as shown in Figure 4.

Figure 4– The schematic showing the setup for the Digital Output test

On the Digital I/O tab, notice that initially the port is configured to be all input. Under Select

Direction click on the bubble above 0 along the output row. You will then see a slider switch

appear at zero in the Select State section. Hit start and watch the LED turn on because the slider

is set to 0. Toggle the switch to observe the LED turn on and off. Figure 5 shows what the test

panel should look like. Click “Close” to close the test panels.

Figure 5– Digital I/O Test Panel.

What to turn in:

1. Did the waveform match your expectation?

2. Did the LED light up when you toggled the switch?

Section 2 – LabVIEW working environment

Objectives: By the end of this section the student will:

1) Be familiar with the LabVIEW environment, creating sub VIs and llb libraries

2) Learn about different data types in LabVIEW and debugging techniques

3) Review properties of panels

Background Information

Unlike text-based programming, LabVIEW is a graphical programming language that uses icons

and node-to-node data flow programming. This section describes key concepts including the

LabVIEW environment, Data Types, Data Flow Programming, Debugging techniques, useful

shortcuts and tools, and the NI Example Finder in LabVIEW, which are addressed below.

The LabVIEW Environment: LabVIEW programs are called Virtual Instruments (VIs).

Controls provide the inputs and Indicators display the outputs. Each VI is made up of a

Front Panel that provides the user interface and a Block Diagram that represents the

program. Figure 6 shows a front panel and block diagram for a temperature sensor VI.

Figure 6– Example showing the front panel and block diagram windows. [E80 Lecture on March

5, 2008 - www.eng.hmc.edu/.../LabView%20and%20Matlab%20Lecture-S08-final-%20RM.ppt]

In LabVIEW, you draw a block diagram describing the relationship between controls and

indicators in your VI. Then you arrange these controls and indicators on a front panel.

You interact with the front panel when the program is running. You can control the program,

change inputs, and see data updated in real time. Controls are used for inputs such as adjusting a

slide control to set an alarm value, turning a switch on or off, or stopping a program. Indicators,

such as graphs, thermometers, and lights, display output values from the program.

Every front panel control or indicator has a corresponding terminal on the block diagram. The

block diagram also contains various functions connected by wires.

One has access to a number of tools and palettes in LabVIEW. The following figures from

National Instruments’ Introduction to LabVIEW tutorial describe the basic palettes and panels

one would use in LabVIEW.

Figure 7 – Controls Palette

Figure 8 – Functions Palette

Figure 9 – Tools Palette

Figure 10 – Status Toolbar

Data Types in LabVIEW: LabVIEW uses many data types, including Boolean,

numeric, arrays, strings, and clusters. The color and symbol of each terminal indicate the

data type of the control or indicator. Control terminals have a thicker border than

indicator terminals. Also, arrows appear on front panel terminals to indicate whether the

terminal is a control or an indicator. An arrow appears on the right if the terminal is a

control and on the left if the terminal is an indicator. Figure 11 shows the various data

types present in LabVIEW. Most basic data types such as integers and complex numbers

are self explanatory. Arrays group data elements of the same type. An array can have one

or more dimensions and as many as (231

) – 1 elements per dimension, memory

permitting. Clusters group data elements of mixed types.

Figure 11 – Common data types found in LabVIEW

Understanding Data Flow Programming: LabVIEW follows a data flow model for

running VIs. A block diagram node executes when all its inputs are available. When a

node completes execution, it supplies data to its output terminals and passes the output

data to the next node in the dataflow path. Thus it is important to program node to node

as opposed to “line by line”.

Common Debugging Techniques: When your VI is not executable, a broken arrow is

displayed in the Run button in the palette. Here are some common debugging techniques

that allow you to step through your block diagram and identify possible errors, also

described in figure 12.

1. Finding Errors: To list errors, click on the broken arrow. To locate the bad object,

click on the error message, or show error.

2. Execution Highlighting: Traces the flow of the data, allowing you to view

intermediate values. Click on the light bulb on the toolbar.

3. Probe: Used to view values in arrays and clusters. Click on wires with the Probe tool

or right-click on the wire to set probes.

4. Retain Wire Values: Used with probes to view the values from the last iteration of the

program.

5. Breakpoint: Sets pauses at different locations on the diagram. Click on wires or

objects with the Breakpoint tool to set breakpoints.

Figure 12 – Common debugging techniques in LabVIEW

Short-cuts and important tools: A few of the important shortcuts are listed below:

1. Ctrl-E: Allows you to shift between the front panel and the block diagram

2. Ctrl-B: Allows you to delete broken wire

3. Ctrl-U: Cleans up the block diagram. This re-orders functions, wires and other objects

on the block diagram in a cleaner, more organized manner. This is good for small VIs

but does not work too well for large VIs

4. Ctrl-Z: Undo

5. Ctrl-H: The Amazing Context Help Window displays basic information about

LabVIEW objects when you hover the cursor over each object, as shown in Figure

13. Objects with context help information include VIs, functions, constants,

structures, palettes, properties, methods, events, and dialog box components.

Figure 13 – Using the context help window

SubVI: After you build a VI, you can use it in another VI as a function. A VI called from

the block diagram of another VI is called a subVI. You can reuse a subVI in other VIs.

To create a subVI, you need to build a connector pane and create an icon.

Finding Examples in LabVIEW: LabVIEW offers a large number of Example VIs that

are saved on your computer with the installation of LabVIEW. These example VIs may

be used as a starting platform for many projects. You use the NI Example Finder, shown

in Figure 14 to find these examples. From the Getting Started screen, clicking on Find

Examples will lead you to the Example Finder. From the block diagram of a VI, clicking

on help and selecting Find Examples… will lead you to the Example Finder.

Figure 14 – The NI Example Finder

Exercise 2 – Getting familiar with the LabVIEW environment and creating a

sub VI

Problem statement – Create a sub VI that adds two inputs and outputs the sum.

1. Launch LabVIEW by double-clicking the icon on the desktop or by selecting

Start » Programs » National Instruments » LabVIEW 2009 » LabVIEW.

Open a new Blank VI from the Getting Started screen (<Ctrl-N> opens a new VI.)

2. Place the Add function on the block diagram. Right-click on the block diagram and navigate to

Programming (You may have to click on the double arrow at the bottom of the palette to

expand the menu) » Numeric » Add.2

2 It is a hassle to click on the double arrow each time you need a Programming element. To make

Programming show up by default, right click on the block diagram, click on the pin on the upper

left corner of the Functions palette, and select View » Change Visible Categories… Check the

Programming box and click OK.

3. Create controls and indicators by right-clicking on the adder icon and selecting Create »

Control or Indicator. To make the controls and indicators small, right-click on them and

remove the checkmark on View as Icon. To make the diagram neater, click on Edit » Clean Up

Diagram. Rearrange the front panel as you see fit. The block diagram and front panel should

resemble Figure 15.

Figure 15 –The block diagram and front panel describing what your VI should look like.

4. Now we will create the connector nodes of the subVI. On the front panel right-click the icon at

the top right and select Show Connector to reveal the connector pane. Then right-click the

connector pane and select patterns. Choose the pattern with two input terminals and a single

output terminal. Figure 16 and 17 shows the process

Figure 16 – Accessing the connector pane

Figure 17 – Selecting the required pattern

5. Assign icon terminals to the two controls and the indicator by first left-clicking on an icon

terminal on the connector pane and then clicking the desired control/indicator. The connector

will turn from white to orange in color.

Note: The general convention is to have controls as data inputs on the left side and indicators as

outputs on the right side of this icon.

6. Now we will change the icon representing the subVI to one that is more descriptive of its

function. Right-Click on the connector pane and select Edit Icon... This should open the icon

editor depicted in Figure 18.

Figure 18 – Accessing the icon editor

7. Modify the graphics to more accurately represent the function of the subVI, in this case

Addition as shown in Figure 19. Right-click on the connector pane and click Show Icon to see

the modified icon.

Figure 19 – Modified Icon

8. Save the subVI in a common Labview Library (LLB) in your LabVIEW folder. Click File »

Save. In the bottom right corner of the save dialog, click on New LLB. Name the new LLB

Tutorial. Name the VI adder.

If you wanted to use this subVI in another block diagram you would right-click on the block

diagram and select Select a VI… and choose your adder.vi from the Tutorial library. In Section

4 you will modify and add a more interesting subVI to your block diagram.

What to turn in

1. A print out of the Block Diagram.

Section 3 – Recap: Elements of typical programming

Objectives: By the end of this section the student will:

1) Review LabVIEW programming, such as loops, functions, decision making structures,

and File I/O

Background Information

This section describes key concepts including loops, functions, making decisions, and File I/O in

LabVIEW, which are addressed below.

Loops: Both the While and For loops are located on the Functions » Programming »

Structures palette. The For loop differs from the While loop in that the For loop

executes a set number of times. A While loop, on the other hand, executes the sub

diagram until the conditional input terminal receives a specific Boolean value. The

default behavior and appearance of the conditional terminal is Stop If True. Both loops

have an iteration output terminal, which contains the number of completed iterations. The

iteration count always starts at zero. During the first iteration, the iteration terminal

returns 0. Figure 20 shows how one would draw a loop in LabVIEW.

Figure 20 – Drawing loops in LabVIEW

One important property of loops is the Shift Register. The shift register allows a loop,

such as the For or While loop, to carry forward values from one iteration into the next. It

may be added to a loop by right-clicking on the right or left side of the loop/structure and

selecting Add Shift Register as shown in Figure 21. Adding a control or constant to the

left block of the shift register allows you to place an initial value for the first iteration.

Adding an indicator to the right block of the shift register allows you to view the entire

set of data carried forward after all iterations are done.

Figure 21 – Adding a shift register.

For most LabVIEW VIs you may develop, timing of loops will be important. The

following are a few common timing functions used in LabVIEW.

Time Delay: The Time Delay Express VI delays execution by a specified number of

seconds. For example when placed in a While loop, the while loop does not iterate

until all tasks inside of it are complete, thus delaying each iteration of the loop by the

specified timed delay. This function can be found at Functions » Programming »

Timing » Time Delay.

Wait Until Next ms Multiple: This function waits until the value of the millisecond

timer becomes a multiple of the specified millisecond multiple to help you

synchronize activities. You can call this function in a loop to control the loop

execution rate. However, it is possible that the first loop period might be short. This

function makes asynchronous system calls, but the nodes themselves function

synchronously. Therefore, it does not complete execution until the specified time has

elapsed. This function can be found at Functions » Programming » Timing » Wait

Until Next ms Multiple.

Functions: There are 3 Types of Functions in LabVIEW, illustrated in Figure 22.

Figure 22 – Types of Functions in LabVIEW.

LabVIEW 7.0 introduced a new type of subVI called Express VIs. These are interactive

VIs that have a configuration dialog box that helps the user customize the functionality of

the Express VI. LabVIEW then generates a subVI based on these settings.

Functions are the building blocks of all VIs. Functions do not have a front panel or a

block diagram.

The various types of functions that are available include, but are not limited to:

1) Signal and data simulation

2) Real signal acquisition using the DAQ

3) Instrument I/O assistant

4) Statistics

5) Signal processing

6) Basic math and logic

7) File I/O and writing to files

The SEARCH Button: The functions palette is filled with hundreds of VIs and

functions. The easiest way to identify a set of functions that may be useful for a particular

task you wish to perform is to click on SEARCH on the top row of the functions palette

and enter a particular keyword, as shown in Figure 23. You may then study the narrowed

list of functions, pick the one you desire, and click and drag it onto the block diagram.

You may also determine the palette of the function by double-clicking on it. The first

time you do a search, labVIEW takes a noticeable time to populate the list of functions to

search through.

Search does make it easy to locate VIs or functions with which one is familiar, however

if one were looking for VIs to suit a particular purpose, browsing through the palettes

narrowing down VIs by function would be a better option to using the Search feature.

Figure 23 – The Search Window

Making Decisions in LabVIEW: Often while programming one is required to develop

routines that allow for decision making. Two useful functions for decision making in

LabVIEW are listed below.

1. Case Structure: The case structure has one or more sub diagrams, or cases, one of

which executes when the structure executes. The value wired to the selector terminal

determines which case to execute and can be Boolean, string, integer, or enumerated

type. Right-click the structure border to add or delete cases. Use the Labeling tool to

enter value(s) in the case selector label and configure the value(s) handled by each case.

You may also rename cases by double-clicking on the label. Remember, if you have more

than two cases, it is important to name one of the cases as the ‘Default’ case. For example

if you had three cases that were to be controlled by the integers 1, 2, and 3, you would

have to have one default case that would execute if an unidentifiable control were

inputted into the selection tool. The Case Structure is found at Functions »

Programming » Structures » Case Structure.

2. Select: Returns the value wired to the t input or f input, depending on the value of s. If

s is TRUE, this function returns the value wired to t. If s is FALSE, this function returns

the value wired to f. It is found at Functions » Programming » Comparison » Select.

A few examples are shown in Figure 24.

Figure 24 – Decision making tools in LabVIEW.

• Example A in Figure 24: Boolean input - If the Boolean input is TRUE, the true case

executes; otherwise the FALSE case executes.

• Example B in Figure 24: Numeric input. The input value determines which box to

execute. If the input is out of range of the cases, LabVIEW chooses the default case.

• Example C in Figure 24: When the Boolean passes a TRUE value to the Select VI, the

value 5 is passed to the indicator. When the Boolean passes a FALSE value to the Select

VI, 0 is passed to the indicator.

File I/O or writing to a file in LabVIEW: In LabVIEW, you can use File I/O functions

to:

Open and close data files

Read data from and write data to files

Read from and write to spreadsheet-formatted files

Move and rename files and directories

Change file characteristics

Create, modify, and read a configuration file

Write to or read from LabVIEW measurement files

It is advisable to use the higher level of abstraction of File I/O functions, usually express VIs.

A few are:

Write To Spreadsheet File – Converts a 1D or 2D array of strings, signed integers,

or double-precision numbers to a text string and writes the string to a new csv file or

appends the string to an existing file. The created spreadsheet file may be opened

with Microsoft Excel. This function can be found at Functions » Programming »

File I/O » Write to Spreadsheet File.vi.

Read From Spreadsheet File – Reads a specified number of lines or rows from a

numeric text file beginning at a specified character offset and converts the data to a

2D double-precision array of numbers, strings, or integers. Can be used to read from

Excel files. This function can be found at Functions » Programming » File I/O »

Read From Spreadsheet File.vi.

Write To Measurement File – Express VI that writes data to a text-based

measurement file (.lvm) or a binary measurement file (.tdm or .tdms). This function

can be found at Functions » Programming » File I/O » Write to Measurement

File.vi.

Read From Measurement File – An Express VI that reads data from a text- based

measurement file (.lvm) or a binary measurement file (.tdm or .tdms) format. You can

specify the file name, file format, and segment size. This function can be found at

Functions » Programming » File I/O » Read From Measurement File.vi.

These functions are easy to use and excellent for simple applications. In the case where

you need to constantly stream to the files by continuously writing to or reading from the

file, you may experience some overhead in using these functions.

Exercise 3 – Using elements of typical programming

Problem statement – Create a VI that

1. Uses a bandpass filter to filter uniform white noise and generate a range of frequencies

2. Analyzes the filtered signal using an fft spectrum

3. Measures the dominant frequency of the filtered signal as a function of time

4. Writes the measured values to a spreadsheet file

This exercise is a contrived problem aiming to introduce you to the variety of functions available

in LabVIEW.

1. We will first generate white noise continuously. Draw a while loop on the block diagram.

Search for the Uniform White Noise Waveform VI using the Search button on the upper right

corner of the Functions Palette (which appears by right-clicking on the block diagram) or

through Functions » Programming » Waveform » Analog Waveform » Waveform

Generation » Uniform White Noise Waveform.vi. The White Noise function generates 1000

samples at 1 kHz by default. Generate uniform white noise continuously by placing the Uniform

White Noise Waveform VI in the while loop. To graph the result you will have to place a graphic

indicator (waveform) on the front panel of the VI, by right-clicking on the Front Panel and

selecting Graph Indicator. You may then connect the output terminal of the function you choose

to the graph on the block diagram.

2. Now we shall filter the signal generated by the Uniform White Noise Waveform.vi. Place the

Filter Express VI (Functions » Express » Signal Analysis » Filter) in the while loop. As it is an

Express function, a screen with various options regarding the VI should pop up when you place

the VI on the block diagram. You may access this screen at any time by double-clicking on the

VI. Create a 10th

order IIR Butterworth bandpass filter with a lower cut-off of 300 Hz and an

upper cut-off of 310 Hz. Also create a front panel control for user-configurable upper and lower

cutoff frequencies to override the defaults you just entered. You may encounter a LabVIEW bug

such that the upper cutoff node does not appear on the filter icon. If so, double-click on the

Filter Express VI, then choose OK and the node should appear. Your block diagram should look

similar to that in Figure 25. To graph the result you will have to place a graphic indicator

(waveform) on the front panel of the VI, by right-clicking on the Front Panel and selecting Graph

Indicator. You may then connect the output terminal of the function you choose to the graph on

the block diagram.

Figure 25 – The Filter Express VI with controls. The Brown wire represents the incoming

Uniform White Noise Signal and the Blue wire represents the filtered signal

3. We will now plot the FFT spectrum of the filtered signal. First place the Spectral

Measurements Express VI (Functions » Express » Signal Analysis » Spectral) shown in Figure

26 on the block diagram. On the properties screen of the Express VI, let the Selected

Measurement be Magnitude (Peak) and the Result be in dB. Use a Hanning window. Connect the

input to the filter output. Add another graph indicator to display the magnitude spectrum on the

Front Panel. Re-label the X-axis on the graph to “Frequency”.

Figure 26 – The spectral Measurements Express VI with a graphic indicator.

4. Next we will find the dominant frequency of the filtered data using the Tone Measurements

Express VI as shown in Figure 27. Search for the Tone Measurements Express VI using the

Search button on the upper right corner of the Functions Palette (which appears by right-clicking

on the block diagram) or through Functions » Programming » Waveform » Analog

Waveform » Waveform Measurements » Tone Measurements Express VI. Select frequency

as the single tone measurement. Also create an indicator for the output of the Tone

Measurements VI, by right-clicking on the output node and selecting Create » Numeric

Indicator.

Figure 27 – The Tone Measurements Express VI measuring the dominant frequency of the

Filtered Signal.

5. Now we shall compare the dominant frequency of the filtered signal to a user-input limit (307

Hz). Place an LED on the front panel by right-clicking on the front panel, selecting LEDs, and

choosing an LED of your choice. Next place the Greater? VI on the block diagram by right-

clicking on the block diagram and selecting Programming (you may have to click on the double

arrow at the bottom of the palette to expand the menu) » Comparison » Greater?

Now connect the output of the Tone Measurements VI to the upper node of the Greater? VI.

Create a control to the lower node of the Greater? VI by right-clicking on the node and selecting

Create » Control. Finally connect the output of the Great? VI to the LED. Thus if the frequency

is over that limit (307 Hz), the LED on the front panel will light up. If not, the LED stays off.

Your code should be similar to that in Figure 28.

Figure 28 – The spectral Measurements Express VI with a graphic indicator.

6. We will now write the dominant frequency to an array. First place the Build Array function

(Functions » Programming » Array » Build Array) on the block diagram, in the while loop.

Drag the bottom of the function down, so as to have two rows. Connect the output of the Tone

Measurements VI to the bottom row of the Build Array function. Now create a shift register by

right-clicking on the right edge of the While loop and selecting Add Shift Register. Now

connect the shift register on the left edge of the loop to the top row of the Build Array function.

Finally connect the output of the Build Array function to the shift register on the right edge of the

While loop. Your code should look similar to that in Figure 29.

Figure 29 – The Build Array function and the convert dynamic data VI .

Note: Usually when appending dynamic data to an array, LabVIEW will automatically use the

Convert from Dynamic Data.vi to convert dynamic data to a 1D array. If this doesn’t happen and

you don’t see the icon displayed above you should add the Convert from Dynamic Data.vi to

your block diagram and connect the dynamic data to the VI prior to appending it to the bottom

row of the build array function as seen in Figure 29.

7. Now write the array to a spreadsheet file using the write to spreadsheet VI. Search for the

write to spreadsheet VI (Functions » Programming » File I/O » Write to Spreadsheet File.vi)

and place the VI outside the while loop (ask yourself why?). Wire the output node of the shift

register on the right side of the loop to the 1D data node on the Write to Spreadsheet VI. Now

create a constant for the transpose? node on the Write to Spreadsheet VI and set the constant to

be T (True). Furthermore, you want the shift registers to start at zero every time you run the VI.

Therefore right-click the input node of the shift register on the left side of the loop and create a

constant, as shown in Figure 30.

Figure 30 – Shift register output node connected to the write to spreadsheet file VI and the input

node of the shift register connected to the initial value constant.

8. Attempt to Run the VI. You will see that you are unable to do so due to the broken Run arrow.

Debug the problem by clicking on the broken Run arrow. Correct the error by adding a control

to the conditional terminal of the While loop.

After correcting the error, click on the Light Bulb icon on the toolbar (Next to the Pause

Button). This will allow you to step through your code as it moves from node to node on the

block diagram. Now Run the VI. Once you are satisfied by the results you receive at each node

on the block diagram, de-select the Light Bulb icon and move to the front panel. On the front

panel you should see the LED flash on and off occasionally. After a bit stop the VI. When

prompted to save the results, save them as results3. Open the spreadsheet with MS Excel and

verify the various dominant frequencies recorded. Are they what you expected to see?

9. Save your VI in the folder you created for the tutorial in exercise 2.

What to turn in:

1. A print-out of the Block Diagram and the Front Panel.

2. Did the spreadsheet file contain values between 300 and 310 Hz?

Section 4 –Instrument Control

Objectives: By the end of this section the student will:

1) Control instruments through GPIB and Serial Ports

Background Information

LabVIEW provides device drivers to control a wide variety of commercial instruments over

GPIB, Serial, USB, and Ethernet ports. In this tutorial, you will generate a Bode plot for an RC

circuit using a signal generator and an oscilloscope controlled by LabVIEW. You will control

the signal generator over a serial port and the oscilloscope over the General Purpose Interface

Bus (GPIB).

Serial: The RS-232 serial communication port sends data one bit at a time over a single

communication line to a receiver. Therefore this method works best when data transfer

rates are low or when data must be transmitted over long distances.

The keys to successful serial communication are to match the physical and timing

interfaces. Despite the fact that only one wire is carrying data in each direction, standard

serial cables have either 9 or 25 pins. Moreover, they come in male and female flavors.

Worse yet, you may occasionally encounter a null-modem cable that swaps the transmit

and receive pins between the two ends. Take care to obtain the appropriate cable to

connect the device to your computer.

A data transmission consists of one start bit followed by 5-8 (usually 8) data bits

followed by an optional parity bit and 0-2 optional stop bits. Both the transmitter and

receiver must agree on how many data, parity, and stop bits to expect. Check the manual

for your device.

Serial links don’t use an explicit clock; instead, both sides agree on the speed, called the

baud rate, of communication. For example, 9600 baud indicates 9600 bits per second. If

the system is set to 1 start, 8 data, 1 parity, and 0 stop bits, then each character requires

10 bits, so the system transmits 960 characters per second.

Most computers today no longer have built-in serial ports. National Instruments sells

USB-to-RS232 cables like the one shown in Figure 31 with a built-in converter for

speaking to older devices with RS232 ports.

Figure 31 – The NI USB to RS 232 cable.

[http://sine.ni.com/nips/cds/view/p/lang/en/nid/12844]

The Art of Electronics by Horowtiz and Hill has a helpful section on troubleshooting

serial links.

GPIB: Stands for General Purpose Interface Bus as defined by the ANSI/IEEE standard

488.1-1987 and 488.2-1992. It is a common interface of communication used by various

instrumentation companies. Communication occurs through a digital 8-bit parallel

interface with transfer rates of 1 MB/s or more. For more information visit

http://en.wikipedia.org/wiki/IEEE-488. The cable you will use for the tutorial is shown in

Figure 32. GPIB is being replaced by Ethernet and USB in most new instrumentation but

is still widely found on older devices.

Figure 32 – GPIB to USB-B GPIB cable. [http://sine.ni.com/psp/app/doc/p/id/psp-354]

Bode Plots: In this section it is important that you understand how a first-order low-pass RC

filter works. Furthermore you will be required to understand the Bode plot (or at least the gain-

magnitude frequency response) associated with a first-order RC low-pass filter. Recall that a

first-order low-pass system has a transfer function of

H(j) = 1/(1+j)

where = RC is the time constant.

Some characteristics of the Bodeplot of a first-order low-pass filter are listed below and shown in

Figure 33.

1. The gain, in units of decibels (dB), is 20 log (Vin/Vout)

2. The corner frequency occurs at -3 dB

3. The slope of the drop off line is -20 dB/decade.

Although a Bode plot technically refers to both the gain and phase portions of the frequency

response, we will only consider the gain in this tutorial.

Figure 33 – Bode plot of a first-order RC Low-Pass Filter. The image was taken from

Electronics Tutorial: Passive Low Pass Filters - http://www.electronics-

tutorials.ws/filter/filter_2.html

Equipment required for this section:

1. Agilent 33250A Signal Generator

2. HP 54600A oscilloscope

3. Resistor (2.2 k)

4. Capacitor (10 nF)

5. Protoboard

6. The appropriate connectors

Exercise 4 – Controlling a signal generator and oscilloscope

Problem Statement – Design a VI that controls a signal generator and oscilloscope to make a

Bode plot of a first-order low-pass filter.

1. What is the theoretical cut-off frequency of the first-order RC filter shown in Figure 34?

2. Build the RC circuit on a protoboard and connect the signal generator and oscilloscope to the

respective terminals. Let channel 1 of the oscilloscope measure the unfiltered input to the RC

circuit and channel 2 measure the filtered output.

Input Signal Output Signal2.2 kW

10 nF

Figure 34 – Schematic of a first order RC filter.

3. Turn on the oscilloscope and the signal generator. Be sure the waveform generator is in High

Z mode. This can be found under Utility » Output Setup » Load » High Z. Plot both channels

on the oscilloscope and observe the amplitude of the waveforms as you increase the frequency.

Measure the peak to peak amplitudes of the waveforms at 1, 2, 5, 10, 20, 50, and 100 kHz and

draw a Bode plot by hand. If it does not match your expectations, debug until it does.

4. To automatically generate a Bode plot with LabVIEW, you will want to take measurements at

points logarithmically spaced between a start and stop frequency, in a manner analogous to the

Matlab logspace command. Since LabVIEW lacks such a function, we have written a subVI for

you. Copy Logspace.vi from \\charlie.hmc.edu\Clinic\Engineering\tutorials\LabVIEW to your

folder. Open a new VI in LabVIEW and add the Logspace.vi as a sub VI to the block diagram.

You may do this by right-clicking on the block diagram, selecting Select a VI… and choosing

Logspace.vi from your folder. Logspace.vi takes in the following inputs: upper bound and lower

bound of the frequencies in which you will measure the response of the filter and the number of

data points per decade. Create constants for the respective nodes. It is advisable that you choose

an upper bound of 100 kHz or higher and a lower bound of 100 Hz. Your code should look

similar to that in Figure 35.

Figure 35 – Logspace.vi with controls and an indicator.

5. You will now need to install the instrument drivers required for this exercise. Select Tools »

Instrumentation » Find Instrument Drivers or Help » Find Instrument Driver. Choose

Agilent Technologies as the manufacturer and then click Search. Choose the folder labeled

ag33xxx Instrument Driver and install the driver for LabVIEW 2009. You will have to register

on NI.com to be able to download the driver. One would repeat the same steps as above to obtain

the drivers for the oscilloscope, however as the updated drivers have errors we will use the old

drivers present in the Labview folder on Charlie. Copy the folder labeled hp546xxx on to your

desktop from \\charlie.hmc.edu\Clinic\Engineering\tutorials\LabVIEW.

6. Connect the waveform generator to the PC via the NI USB-232 Serial to USB adapter. Note

that you will need a 9-pin null modem gender mender that should come with the adapter.

Determine which COM port to which the waveform generator is connected by clicking on

Control Panel » System » Hardware » Device Manager and looking at the port assignments.

For instance the NI USB-232 may show up on COM4. Connect the oscilloscope to the PC via the

NI GPIB USB-B adapter. This will normally be assigned to port GPIB1::1::INSTR.

7. The block diagram will have three subsections:

control of the Signal Generator

control of the Oscilloscope

Bode plot calculations

8. You should save the VI, through the Save As option, in the folder you created for this tutorial

prior to proceeding.

Subsection 1 – Control of the Signal Generator

1. First we shall draw a loop to iterate through the frequencies outputted by the Logspace.vi.

Draw out a For Loop on the block diagram but do not include the Logspace.vi in the For

Loop.

2. Next we will draw a sequence that will divide the block diagram in the For loop into

three consecutive steps. Draw a Flat Sequence (Functions » Programming » Structures

» Flat Sequence) on the block diagram in the For Loop.

3. We will then convert a driver into a subVI that we can use in our VI. Right-click on the

block diagram and select Select a VI » My Computer » Local Disk (C:) » Program

Files » National Instruments » LabVIEW 2009 (2010 if that is the current version on

your laptop) » instr.lib » Agilent 33xxx Series » Examples » Generate Standard

Waveform.vi and place it in the flat sequence. You will notice that the example driver

does not have any nodes on it. Double-click on the Generate Standard Waveform.vi and

you will see the front panel of the driver open. We will now modify the subVI to include

three nodes. Right-click on the icon of the subVI present on the top right corner of the

window and choose Show Connector as shown in Figure 36. As you did in Section 2 of

the tutorial, create input connections for VISA Resource Name, Frequency and

Amplitude. Save the subVI and close the block diagram window of the subVI. Now if

you return to the main block diagram you will see the subVI has three nodes.

Figure 36 –Modifying the sub VI by first accessing the connector pins

4. Now we will provide inputs to the Waveform Generator subVI. Next we will identify the

Resource Name associated with the Waveform Generator. Create a Visa Resource Name

Control, by right-clicking on the VISA resource name node of the Generate Standard

Waveform VI and selecting Create » Control. Place the control outside the For loop and

then wire it to the Standard Waveform VI. Also create a control for the Amplitude node

of the Generate Standard Waveform VI, place the control outside the For loop, and set

the amplitude to be a constant of 5 V. Wire the frequency output node of the Logspace.vi

to the respective input node on the Standard Waveform VI.

5. Set the amplitude to 5 V on the front panel. Set the VISA Resource name to the COM

port representing the waveform generator. Now run the VI. You should see and hear the

waveform generator stepping through the frequencies you requested. Note: When

attempting to run your VI, LabVIEW may ask you to locate sub VIs required for the

Generate Standard Waveform.vi. Locate them in the folder Agilent 33xxx Series. Don’t

forget to search all folders within for the requested files. You may also obtain an error

stating that the dependency was loaded from a new path. You may ignore the error.

Subsection 2 – Control the Oscilloscope

1. First we will modify the block diagram by expanding the Flat Sequence. This will allow

us to accommodate the functions controlling the oscilloscope. Add two additional frames

to the flat sequence by right-clicking on the boundary of the flat sequence and selecting

Add Frame After. The second frame will be used to call the auto scale function of the

oscilloscope to scale the recorded signals. The third frame will be used to call a function

that reads the measurements made by the oscilloscope for each channel.

2. Add the HP546xxx Autoscale.vi to the second frame of the flat sequence from the

hp546xxx folder. Create a control for the VISA Session node and place it outside the For

Loop. On the front panel, set the VISA Session control to GPIB.

3. The third frame will contain two HP54600A/610B Measurement VIs to measure the

amplitudes of the filter input and output. The input amplitude should be 5 V, but it

doesn’t hurt to measure as a sanity check. Connect the VISA session nodes of the two

Measurement VIs to the same GPIB VISA session. Create controls for the source node

and function node of each measurement. The source node selects the oscilloscope

channel. The function node specifies the kind of measurement, such as peak to peak

voltage. Create an indicator for each function-reading output node.

4. On the front panel, you want to set the source of the first measurement to channel 1 and

the function to V p-p. Set the second measurement to channel 2 V p-p. Your code so far

should look similar to Figure 37

Figure 37 – The various subVIs your code should use to call measurements made by the

oscilloscope

5. Run the VI and observe the readings made by the oscilloscope.

Subsection 3 – Developing a Bode plot

1. First lay out the code required to calculate the Gain of the filter. Outside the flat sequence

(but in the For loop), determine the Gain of the system using the divide function

(Functions » Programming » Numeric » Divide). Determine the Log of the Gain by

using the Logarithm Base 10 function (Functions » Mathematics » Elementary and

Special Functions » Exponential Functions » log10). Multiply (Functions »

Programming » Numeric » Multiply) the output by 20 to obtain the Gain in dB. Your

code should look similar to that in Figure 38.

Figure 38 – Code describing the calculation of the gain in dB

2. Now we will group the Gain and Frequency results together so as to plot a Waveform

graph. Place the Bundle function (Functions » Programming » Cluster, Class &

Variant » Bundle) on the block diagram outside the For loop. Drag it down until you

have two rows. Connect the Gain to the bottom row of the Bundle and the output of the

Logspace.vi to the top row of the Bundle. The bundle allows you to combine two

variables, say X (top row) and Y (bottom row) into a single waveform prior to connecting

it to a XY graph.

3. Now we will build the Waveform graph. Shift to the Front Panel Window, right-click,

and place an XY Graph (Right-click » Graphic Indicators » XY Graph) on the Front

Panel. Right-click on the graph, select Properties » Scales » X-axis (on the drop down

menu) » Log (check the box). Also re-label the X-axis Name to frequency. (On shifting

back to the block diagram you might have to delete the express VI and keep the Graphic

indicator).

On the block diagram connect the Output Cluster (from the bundle) to the waveform

graph. Your code for this section should look like that in figure 39. Also remember on the

front panel you would like the graph to be in log scale.

Figure 39 – The code required to generate a Gain Magnitude frequency response plot

4. Run your VI. At what dB do you observe the cut-off frequency? Is it what you expected?

What is the slope of the line that drops off rapidly beyond the cut off frequency? Is it

what you expected?

What to turn in:

1. A screenshot of the Front Panel displaying the controls and the Bode plot, plotted using at

least 5 points per decade.

2. A printout of the Block Diagram.

Works Cited:

1. Two exercises were modified renditions taken from the NI tutorial. Furthermore a number of

slides present in the Background Information sub-part of each section were also grabbed from

the tutorial available at http://zone.ni.com/devzone/cda/tut/p/id/5247.

2. The shift register diagram was taken from

http://learnlabview.blogspot.com/2008/06/programming-labview-shift-register.html.

3. The pictures of the GPIB and Serial cable were taken from NI.com

http://sine.ni.com/psp/app/doc/p/id/psp-354 and http://sine.ni.com/images/products/us/ni-

9870_w_cable_l.jpg

4. E80 lecture Feb 5, 2008, www.eng.hmc.edu/.../LabView%20and%20Matlab%20Lecture-S08-

final-%20RM.ppt.

Good Reference Sources:

1. Bishop, Robert H. & National Instruments, LabVIEW 2009 Student Edition, Prentice Hall

2010, ISBN-10: 0-13-214129-9 ISBN-13: 978-0-13-214129-1

2. The LabVIEW youtube channel.

http://www.youtube.com/user/Labview#p/c/498405DC54786398/13/1KT98-7B2E8

3. The LabVIEW wiki

http://labviewwiki.org/Development_Environment

4. If you are interested in learning more about the Mathscript feature

http://techteach.no/labview/lv85/mathscript/index.htm

5. NI.com – It has EVERYTHING! (Well almost)

Feedback:

As this is the first time we are using tutorials as part of the clinic program, we would really value

your feedback.

How many hours did you spend on the tutorial?

Did you face any problems during the tutorial? Please explain

What would you like to see added to the tutorial? Please explain

What do you feel is not required in the tutorial? Please explain