aoe 3054 - dept.aoe.vt.eduaborgolt/aoe3054/manual/inst3/aoe3054_session...• all operations will be...

AOE 3054

Digital Measurements:

Data Acquisition with LabView

1

Instrumentation Lab Schedule

5 periods – every other week (Meet in RAND 15B)

– Session 1 (Week of Feb 5): Analog Instrumentation

– Session 2 (Week of Feb 19): Experiment 6: Analog

(Logbook due)

– Session 3 (Week of Mar 12): Digital Measurements

– Session 4 (Week of Mar 26): Spectral

Analysis/Advanced Programming

– Session 5 (Week of Apr 9): Experiment 6: Digital

(Logbook due)

Goals of the session

• Understand the basics of making the NI myDAQ work for

controlling an experiment

• Build the data acquisition and control techniques needed

to digitally run Experiment 6 in LabVIEW

– Basic scope

– Aliasing

– Calibrating and ultimately controlling the function generator

• IT IS STRONGLY SUGGESTED TO SAVE THE CODES

YOU ARE DEVELOPING/MODIFYING AS OFTEN AS

POSSIBLE 3

Digital Measurements Lab Agenda

1. Experiment 6 Digital introduction

2. Measuring function generator input and beam response

using myDAQ

3. Calibrating function generators

4. Find natural frequency of beam using myDAQ

5. myDAQ Resolution Example

6. Modify Homework codes

4

Experiment 6 Digital Introduction

• In the second Instrumentation Lab (Experiment 6a),

you manually controlled a function generator to

excite a beam and used an oscilloscope to

measure the response of that beam.

• Week 5’s Instrumentation Lab is essentially a redo

of the first Experiment 6, but will incorporate new

digital measurement techniques to automate most

of the data taking.

5

Experiment 6 Digital Introduction

• Specifically, you will be using the myDAQ to

output a voltage signal that will control the function

generator. The myDAQ will also measure the

function generator output as well as the output

from the proximeter.

• All operations will be controlled via Labview, using

a code that builds off of the homework

assignments and will be completed in

Instrumentation Lab 4.

• Further details of Experiment 6 Digital can be

found on the course website. 6




using myDAQ





7

Homework 3 VI

• Your Homework 3 VI’s utilized the 2 analog input

channels on the myDAQ to measure two signals.

• We will use this code again to measure and display two

signals- an excitation signal (function generator) and

response signal (proximeter)

8

Connect Experiment 6 Components

• Connect the Function Generator to the power

amplifier, using a BNC T-connector.

• Connect the Power Supply to the proximeter

and set it up for the correct output voltage.

• Refer to previous lecture slides for

instructions on properly connecting the

devices.9

myDAQ Connections

• Connect your myDAQ to your computer using the USB

port.

• Open LabView and your functional Homework 3 VI (use

the solution if your program had errors)

10

myDAQ Connections

• BEFORE turning any of the equipment on, make

sure that you do not supply more than 20V to

the myDAQ through its analog inputs (myDAQ

Overvoltage protection: +/-30V, 20 Vrms)

• This will require connecting the excitation and

response signals to the oscilloscope and

measuring amplitudes before connecting the

myDAQ.

• A good first step is to turn the function

generator amplitude control all the way down

and ensure the amplitude control is pulled

out to produce voltage between 0 and 2V. 11

myDAQ Connections:

Excitation• Attach a BNC-to-BNC-probe connector to

the T-connector on the function generator

12

To Amplifier

BNC to BNC

probe (to

oscilloscope)

myDAQ Connections: Excitation

• Attach the BNC-to-BNC connection from the function

generator to a T-connector on CH1 of the oscilloscope, and a

BNC-to-clipping-probe connector to the T-connector.

13

From function

generator BNC to

clipping probe

(to myDAQ)

myDAQ Connections:

Excitation• Clip the two ends of

the probe to wires

and attach to the

myDAQ AI0 channel.

Make sure the red

clip goes to the 0+

channel, and black to

the 0-.

14

AI0+ AI0-

myDAQ Connections:

Response• Attach the output

BNC connector of

the proximeter to a

BNC-to-clipping-

probe connector

using a T-connector

• Then connect the

T-connector to

Channel 2 on the

oscilloscope 15

Proximeter

Output

BNC to

clipping probe

(to myDAQ)

To Ch2 on

Oscilloscope

myDAQ Connections:

Response• Clip the two ends of the clipping probe coming from the

proximeter to wires and attach them to the myDAQ AI1

channel. Make sure the red clip goes to the 1+ channel,

and black to the 1-.

16

Final myDAQ Connections:

Excitation+Response

17

Function

Generator

Proximeter

Verify Connections

• Before turning equipment on, verify all of

your connections are correct and set to the

correct voltage. Ask your TA if you have

any questions.

• Then disconnect the four clip-ons

connecting the myDAQ.

• Verify that the function generator is set

to output no more than 2 V.

18

Turn on Equipment

• Once the setup is verified, turn on the function generator,

amplifier, multimeter, and power supply.

• Verify that the signals look good and within range on the

oscilloscope.

• Once you established that both excitation and response are under

20V amplitude, turn all the equipment off.

• Reconnect the myDAQ.

• You can now turn the equipment back on, your myDAQ is ready

for acquisition!

19

Using Homework 3 VI• The Homework 3 VI and subVI should already be

configured to read the correct channels on the

myDAQ.

• Verify this by opening your subVI block diagram.

20

Verify myDAQ Channels• Double click the DAQ Assistant Express VI. The

following window then appears:

21

Verify myDAQ Channels• Click the “Details” button under “Configuration”

22

Verify myDAQ Channels

23

• As expected, the myDAQ is reading the

Excitation signal from channel AI0 and

Response signal from AI1.

Set Test Conditions

24

• Close out of the DAQ Assistant screen and return to the

Main VI front panel.

• Set the amplitude of the function generator signal up to

about 2 V using the “AMPL” knob. Make sure it is pulled

out (because when it is pushed in, it supplies a voltage up

to 20 V).

• Set the frequency of the function generator to about 12

Hz.

Introduction to Aliasing• Set your VI to take 10 samples at a rate of 10

Samples/s. The output should look similar to this:

25

Introduction to Aliasing• Note that even though the excitation signal is set to 12 Hz, the

sampling rate in LabView is too low to accurately resolve this

signal.

• Likewise, the response cannot be accurately characterized either.

• This is known as aliasing, and was introduced in the previous

homework. It will be discussed extensively in the next lecture and

the 4th Instrumentation Lab, and is a major concern in signal

analysis.

Grounding the myDAQ

• Some computers have grounding issues when using the myDAQ’s to measure a voltage,

including older Fujitsu models. This causes noisy looking signals, such as that seen below:

Grounding the myDAQ• To fix this problem, connect a banana to clipping probe cable from

the “GND” of the Power Supply to the “AGND” port of the Analog

Input section of the myDAQ. This leads to a much cleaner signal.

• A picture of an example connection is found on the next slide

Grounding the myDAQ

Power Supply

GND

Increase Sampling Rate

• Stop the program and increase the sampling rate to 1000

Samples/S, and increase the number of samples to 1000.

• Now, at this higher sampling rate, the correct signals can

be determined (see front panel screenshot on next slide).

30

Increased Sampling Rate

31

Find Approximate Natural Frequency

• Adjust the function generator frequency until it reaches near the

natural frequency.

– Note: It will be challenging to settle on the exact frequency. The function

generators are sensitive. As you approach natural frequency, make small

adjustments and let the beam settle for a few seconds after each

adjustment.

• In this scenario, it is easiest to spot the natural frequency using

the Lissajous plot.

• At the natural frequency, the Lissajous plot forms an ellipse with

vertical and horizontal axes.

• Write down the frequency you settled on; we will come back to

this later.32

Front Panel at Natural Frequency

33

Phase difference of 90

degrees

Vertical ellipse.

Turn Off Equipment

• Shut off all of the equipment to prepare for the next

phase of today’s lab.

34

Applications to Dynamic Beam

• Two inputs allow force and displacement voltages to be

measured.

• Voltages converted to force and distance (through coil

and proximeter calibrations).

• Dynamic flexibility and spring constants measured at low

frequencies and the result displayed.

• Data file saved and analyzed (see Homework 3 guide).

35




using myDAQ





36

What else is needed for Exp 6?• We want to write a program that will automatically and systematically

determine some parameters of the vibrating beam system like:

– Finding the stiffness k

– Finding the natural frequency

– Finding the resonant frequency

• This will require the code to vary the excitation frequency and record the

beam response.

• Therefore, we need to find a way to control the function generator from

LabVIEW (we will see later we can use the myDAQ as a function generator

as well).

• The output frequency of the function generator can be controlled by providing

a DC voltage to the generator. The output frequency is linearly proportional to

the DC voltage level.

37

Varying Function Generator Frequency

• The function generator frequency can be varied by sending a DC

voltage to the VCF (Voltage Controlled Frequency) BNC connector on

the front of the function generator.

• The DC voltage signal will be produced by the Analog Output of the

myDAQ.

– Note: the myDAQ can output +-2V or +-10V

• Varying the myDAQ voltage will vary the output frequency of the

function generator.

• To produce a given frequency, we need to know the relationship

between myDAQ DC ouput voltage and the generator output frequency

i.e. we need to calibrate the function generator.

• First, we need to set up the DAQ Assistant to Output an analog signal.

38

Rename Labview VI

• We will now build off of the code used so far in the lab.

• Save your current VI-the main VI from Homework 3-as a

new file (i.e. Name_InstrumentationLab3, etc.)

• We will then add a second DAQ assistant, this time to

generate a DC signal to be fed to the function generator.

39

Configuring the DAQ for Analog Output

From the Block Diagram of your VI, Right click and select Input, DAQ Assistant.40

Place VI on the Diagram

41

Configure the DAQ Assistant…

Select generate signals, this is the output portion of the DAQ42


Select Analog output43


Select Voltage44

Select physical output channel

Select an output port45


Since the signal output is a DC

(i.e. constant) signal, the DAQ

only needs to produce one value

to set the DC level.

Select 1 Sample from the

generation mode, finished!

Note that the terminal

configuration lists only RSE

(Referenced Single Ended). When

the myDAQ is used for its analog

inputs, the terminal configuration

is only capable of different

connection.

If the window below does not show up, change the window size

and the window should appear!

46

Analog Output Terminals

• Connect bare wires to the A0 0 and AGND terminals on the DAQ by

inserting the bare ends of the leads into the appropriate terminals and

tightening the screw with the included screwdriver.

• Connect a BNC-to-clipping-probe connector to the DAQ (RED clip to

the lead for A0 0 and a BLACK clip to the lead for AGND).

• Note that for all of these connections discussed today, either a BNC to

alligator clip or BNC-to-clipping-probe connector will work

47

• Single ended connection implies

that the signal will be produced

between one analog output port

channel and the ground (the

“reference” in RSE)

• The ground channel on the DAQ

is labelled AGND (Analog

GrouND)

DAQ Assistant for Analog Output

Now we need to specify the DC level for the

DAQ. This can be done by attaching a

control to the data input…

48

DAQ Assistant for DC output

• Add a while loop around the DAQ Assistant for continuous

output and then connect a stop button to the stop port on the

DAQ and the stop button on the while loop so that the while

loop will smoothly shut down the DAQ when you press stop.

• You are now ready to connect the myDAQ to the function

generator.49

Process: How to control the function generator?

1. You will use the DAQ DC output to determine the relationship

between the voltage supplied to the function generator and the

frequency output by the generator. This relationship is called

the gain or calibration.

2. To do so, you will connect the DAQ analog output to the function

generator input (VCF port) and use the calibration to control the

generator with the DAQ.

50

Setup: How to control the function generator?

1. Place a BNC T-connector on the VCF input of the function generator.

2. Connect the analog output port you selected to the VCF input of the

function generator using a BNC cable/alligator clip and the two leads in the

myDAQ.

3. On the other side of the T-connector, connect a BNC cable to the

multimeter.

51

BNC cable to

the multimeter

From myDAQ

Setup: How to control the function generator?

4. You will want to connect your voltmeter to the DAQ

as well. Disconnect the banana plug currently in the

multimeter, which connected the power supply

voltage to the proximeter. Insert a new banana plug

to BNC connector and connect the BNC coming

from the function generator VCF input to the

multimeter.

5. Using the LabView VI, vary the DC voltage output

and see how the function generator frequency

responds. Verify that the multimeter voltage

matches the output voltage specified in LabView,

and also specify that the frequency displayed on

the function generators matches the frequency

measured by the AI0 channel in the myDAQ

(displayed as the “Excitation Frequency” on the VI

front panel). 52

From function

generator

VCF input

Power supply

to proximeter

connection

Exercise: Calibrating the function generator

• The frequency of the function generator can be

commanded based upon voltage provided to the VCF

port.

• You will open an Excel file to store information on the

calibration (i.e. record voltage input and frequency output)

• Use the DAQ to control the function generator

53

Calibrating the function generator

• To determine this relationship, you need to do a

calibration.

• You need to record the frequency output for different

voltage inputs:– Make sure your function generator is set to sine wave, with the RANGE

knob set to a 10 Hz order of magnitude.

– Adjust the FREQUENCY knob to output a 14 Hz sine wave before

beginning calibration. This is important as the exact calibration depends

on how close to 14Hz you are at the start.

– Run the DAQ VI and record the output frequency and DC voltage values

from the DAQ for ~10-20 voltages. Adjust the input voltages between +/-

2 V. This should lead to function generator frequencies from about .5 Hz

up to about 27.5 Hz

– You will have to adjust your sampling scheme to obtain accurate

readings.

– Record and plot the resulting data in excel and obtain the linear

relationship, i.e. the calibration.

54

Calibrations

• The calibration curve should be linear and look something like the above.

• The offset and slope will be different depending on your station. It is therefore

important that you use the same station from now on (unless you want to redo a

calibration every time).

• The calibration gives you a relationship between in the input voltage (x) and the

frequency (y) and allows you to write a conversion between frequency and voltage.

• Thus in LabVIEW, the user can enter a frequency y and the VI will convert this value to

voltage using (y-b)/m (if the calibration is y=mx+b) and then send this to the DAQ to

control the function generator. This was the subject of Homework 2. 55

y-intercept will be equal to the

frequency the function generator

was set to before calibrating. In

this image, the function generator

was started deliberately started at

10 Hz. Note that you will be

starting at 14 Hz.

How to control the function generator

with LabVIEW

• At this point, your code is

set to provide a voltage

from the DAQ to the

function generator, which

in turns produces a

frequency output that is

function of its calibration.

56

• Since you know the calibration equation, you will be able to

change the code so that the user inputs a frequency that the

code will convert to a voltage value to be fed to the function

generator.

• You can therefore change the “Offset” control seen above

using the arithmetic operations you learned for Homework 2

and the coefficients of the calibration you just measured.

How to control the function generator

with LabVIEW

• You can use the sub-VI from Homework 2 to modify the

“Offset” control in the current code.

• To confirm you have written your code and performed

your calibration correctly, set a frequency in LabVIEW

and measure the excitation signal on the scope. If you

have done everything correctly, the two should match. 57




using myDAQ





58

Reconnect Equipment

• The myDAQ output voltage can control the output frequency of the function

generator much more precisely than manually adjusting the knob.

• We will now try to more accurately measure the natural beam frequency.

• Disconnect the banana plug and BNC to BNC cable in the multimeter from

the function generator calibration, and reconnect the banana plug linking the

power supply to the proximeter input.

• Return the BNC to BNC cable to the wall rack.

59

Turn on Equipment

• Turn on the power supply and amplifier

• Using the LabView VI, adjust the desired frequency of the

function generator until you obtain a vertical ellipse on the

Lissajou plot.

• See how much closer to a phase of 90 degrees you can

get now that you can dial in a precise value compared to

manually adjusting the frequency knob on the function

generator.

60

Power Down Equipment

• Next, turn off all equipment, disconnect all cables and

wires, and return all equipment to their respective storage

locations.

• Leave two wires, a BNC-to-clipping-probe connector, and

a BNC to BNC connector at your table.

61




using myDAQ





62

Generate Sine Wave with myDAQ

• Next we will go through an example showing the limitations of a

digital signal.

• You have seen that the myDAQ is capable of producing an analog

signal through its analog output.

• So what if we were to try to replace the function generator

altogether with the myDAQ?

• We will have the myDAQ generate a sine wave, and compare that

to a sine wave generated by the function generator

63

myDAQ Resolution

• You have seen in class that the resolution of a

digital to analog converter is a function of its

number of bits and voltage range:

– 16 bit resolution, i.e. 216 = 65536 levels for

distinguishing/generating a signal.

– At an analog output voltage range of -10 to 10 V,

that means that the myDAQ can output voltages

in increments of 20/65536 = 0.3 mV.

64

Download Code

• Download the

OutputSineWave.vi from this

link

• This simple VI uses the

“Simulate Signal” function to

create a sine wave and the

DAQ assistant to output this

sine wave through the analog

AO0 port.65

http://www.dept.aoe.vt.edu/~aborgolt/aoe3054/manual/inst3/OutputSineWave.vi

Connect Devices

• Use a BNC to BNC connector from the MAIN output port on the

function generator to CH 1 on the oscilloscope.

• Connect two wires into the myDAQ for analog output, in the

same manner as described on Slide 47.

• Use a BNC-to-clipping-probe to connect the wire terminals of

the myDAQ to CH 2 on the oscilloscope. Make sure the red clip

corresponds to the AO0 wire, and black clip to the AGND.

66

Device Connections

67

Run VI

• Turn on the oscilloscope and function generator. Set the

function generator to an output frequency of about 4 Hz

with a very low amplitude (always keep in mind the

voltage limit on your DAQ).

• In the VI front panel, set the frequency at 4 Hz and

amplitude to 0.02 V.

• Run the code, and adjust the oscilloscope screen to

accommodate the signals.

68

Oscilloscope Display

69

Function Generator

Signal

myDAQ Signal

Digital Resolution Limitations

• As you can see, the blue myDAQ signal is much choppier and noisier

than the function generator signal.

• While the function generator can output a smooth sine wave, the

myDAQ can only output voltages in increments of 0.3 mV.

• For a low amplitude signal such as this, the 0.3 mV resolution of the

myDAQ can be a significant limitation.

• For higher amplitudes, the myDAQ analog output could actually be

used to directly drive the beam vibration. However, this would require

close synchronization between the myDAQ analog inputs and outputs

which is beyond your current programming capabilities. We will see

more of that in the 4th instrumentation session.70




using myDAQ





71

Modify Homework/Lab 3 Code• Modify today’s code to calculate the spring stiffness as well,

using beam theory.

• You will multiply the excitation signal amplitude by the coil

calibration constant.

• Similarly, you will divide the response signal amplitude by the

proximeter calibration constant.

• You can then divide the forcing amplitude (in Newtons) by the

response amplitude (in meters). At low frequencies, this ratio

is the inverse of the static flexibility and therefore corresponds

to the stiffness k.

• See slide 74 for block diagram screenshot. 72

Note: for those using the station with the -24VDC

supply to the proximeter:

1) There is a voltage divider on the proximeter

drive that divides the output by 2 before you get

to measure it. It is therefore required to multiply

the response signal by 2 to recover the true

displacement voltage.

2) The proximeter calibration is 200mV/mils (as

opposed to 106mV/mils for the older

proximeters).

73

Final Modifications

74

Wrapping Up• You can run your code for various frequencies and find if the

values of k you obtain are consistent with Experiment 6a.

• We now have most of the building blocks for digitally

controlling and measuring the beam response.

• Next lab, we will write code to fully automate the process

and find the resonant frequency of the beam.

• To do this, we’ll have to learn about for loops and advanced

programming techniques (Session 4)

• Make sure you turn off all your equipment, disconnect all the

cables and return them to their original location on the wall.

75

aoe 3054 - dept.aoe.vt.eduaborgolt/aoe3054/manual/inst3/aoe3054_session...• all operations will be...

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