Transcript AOE 3054 - Virginia Tech

AOE 3054
Digital Measurements: Data
Acquisition with LabView
Credits to Borgoltz, Devenport, and Edwards
for some content
1
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
– Storing scope measurements
– Calibrating and ultimately controlling the
function generator
2
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
3
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.
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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.
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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
6
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)
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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.
Remember to take into account whether you are using
the NEW or OLD proximeter.
• Refer to previous lecture slides for
instructions on properly connecting the
devices.
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myDAQ Connections
• Connect your myDAQ to your computer
using the USB port.
• Open LabView and your functional
Homework 3 VI
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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.
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myDAQ Connections:
Excitation
• Attach a BNC-to-BNC-probe connector to
the T-connector on the function generator
BNC to BNC
probe (to
oscilloscope)
To Amplifier
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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.
From function
generator
BNC to
clipping probe
(to myDAQ)
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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-.
AI0+ AI0-
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myDAQ Connections:
Response
To Ch2 on
• Attach the output
BNC connector of
the proximeter to a
BNC-to-clippingprobe connector
using a T-connector
• Then connect the
T-connector to
Channel 2 on the
oscilloscope
Oscilloscope
Proximeter
Output
BNC to
clipping probe
(to myDAQ)
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myDAQ Connections:
Response
• Clip the two ends of the probe coming
from the proximeter to wires and attach to
the myDAQ AI1 channel. Make sure the
red clip goes to the 1+ channel, and black
to the 1-.
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Final myDAQ Connections:
Excitation+Response
Function
Generator
Proximeter
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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.
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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!
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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.
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Verify myDAQ Channels
• Double click the DAQ Assistant Express VI. The
following window then appears:
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Verify myDAQ Channels
• Click the “Details” button under “Configuration”
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Verify myDAQ Channels
• As expected, the myDAQ is reading the
Excitation signal from channel AI0 and
Response signal from AI1.
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Set Test Conditions
• 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.
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Introduction to Aliasing
• Set your VI to take 10 samples at a rate of 10
Samples/s. The output should look similar to this:
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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).
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Increased Sampling Rate
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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.
• 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
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come back to this later.
Front Panel at Natural Frequency
Phase difference of 90
degrees
Vertical ellipse.
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Turn Off Equipment
• Shut off all of the equipment to prepare for
the next phase of today’s lab.
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Applications to Dynamic Beam
• Two inputs allow force and displacement
voltages to be measured.
• Voltages converted to force and distance.
• Dynamic flexibility and spring constants
measured at low frequencies and the
result displayed.
• Data file saved and analyzed.
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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
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What else is needed for Exp 6?
• We need a method for controlling the
Function Generator!
• This will allow us to sweep through a
range of frequencies and automate the
experiment.
• To start understanding how to control the
frequency generator, we will begin by
calibrating it.
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Varying Function Generator
Frequency
• The function generator frequency can be varied by
sending an analog output to the BNC connector on the
front of the function generator.
• The DC voltage signal will be produced by the Analog
Output of the DAQ.
• Varying the DAQ voltage will vary the output frequency
of the function generator.
• Such set-up could be particularly useful to determine the
natural frequency. Calibrating the Frequency Generator
will produce the relationship between myDAQ voltage
and output frequency.
• Now we need to set up the DAQ Assistant to Output an
analog signal.
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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.)
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Configuring the DAQ for Analog Output
From the Block Diagram of your VI, Right click and select Input, DAQ Assistant.
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Place VI on the Diagram
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Configure the DAQ Assistant…
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Select generate signals, this is the output portion of the DAQ
Configure the DAQ Assistant…
Select Analog output
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Configure the DAQ Assistant…
Select Voltage
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Select physical output channel
Select an output port
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Configure the DAQ Assistant…
If the window below does not show up, change the window size
and the window should appear!
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Select 1 Sample from the generation mode, finished!
DAQ Assistant for Analog Output
Now we need to create to input DC signal
for the DAQ. This can be done by attaching
a control to the data input…
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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
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generator.
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
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Process: How to control the function
generator?
1. You will use the DC DAQ output to obtain 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.
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Setup: How to control the function
generator?
1.
2.
3.
Place a BNC T-connector on the VCF input of the function generator.
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.
On the other side of the T-connector, connect a BNC to BNC cable.
From myDAQ
BNC to BNC
connector
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Setup: How to control the function
generator?
4.
5.
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.
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).
From function
generator
VCF input
Power supply
to proximeter
connection
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Exercise: Calibrating the
function generator
• The frequency of the function generator
can be commanded based upon voltage
• 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
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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.
– Run the DAQ VI and record the frequency 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.
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Calibrations
y-intercept will be equal to the
frequency the function generator
was set to before calibrating. In
this image, the function generator
started at 10 Hz. Note that you will
be starting at 14 Hz.
• The calibration curve should look something like this.
• The offset will be different depending on your station.
• This 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
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function generator.
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.
• 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.
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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.
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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
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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.
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Turn on Equipment
• Turn on the power supply and amplifier
• Using the LabView VI, adjust the desired
frequency of the function generator to find
the beam’s natural frequency.
• See how much closer to a phase of 90
degrees you can get, compared to
manually adjusting the frequency knob.
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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.
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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
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myDAQ Generate Sine Wave
• 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
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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 of
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.
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Download Code
• Download the OutputSineWave.vi from the
Scholar site, under Resources->
Instrumentation->Lab 3
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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 48.
• 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.
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Device Connections
66
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.
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Oscilloscope Display
Function Generator
Signal
myDAQ Signal
68
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.
• Consequently, the myDAQ is used to control the
function generator (and automate the acquisition),
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rather than to replace it.
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
70
Modify Homework/Lab 3 Code
• Modify today’s code to calculate the spring
stiffness as well, using beam theory.
• Divide the forcing amplitude (in Newtons)
by the response amplitude (in meters).
Note that this is only valid at low
frequencies.
• See slide 73 for block diagram screenshot.
71
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).
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Final Modifications
73
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 LabView data storage through arrays.
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