AOE 3054 Digital Measurements: Data Acquisition with LabView
Credits to Borgoltz, Devenport, and Edwards for some content 1
Slide 3
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
Slide 4
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
Slide 5
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 5s 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. 4
Slide 6
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. 5
Slide 7
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
Slide 8
Homework 3 VI Your Homework 3 VIs 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) 7
Slide 9
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. 8
Slide 10
myDAQ Connections Connect your myDAQ to your computer using the
USB port. Open LabView and your functional Homework 3 VI 9
Slide 11
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. 10
Slide 12
myDAQ Connections: Excitation Attach a BNC-to-BNC-probe
connector to the T-connector on the function generator 11 To
Amplifier BNC to BNC probe (to oscilloscope)
Slide 13
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. 12 From function generator BNC to clipping probe (to
myDAQ)
Slide 14
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-. 13 AI0+ AI0-
Slide 15
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 14 Proximeter Output BNC to clipping probe (to myDAQ)
To Ch2 on Oscilloscope
Slide 16
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-. 15
Slide 17
Final myDAQ Connections: Excitation+Response 16 Function
Generator Proximeter
Slide 18
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. 17
Slide 19
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! 18
Slide 20
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. 19
Slide 21
Verify myDAQ Channels Double click the DAQ Assistant Express
VI. The following window then appears: 20
Slide 22
Verify myDAQ Channels Click the Details button under
Configuration 21
Slide 23
Verify myDAQ Channels 22 As expected, the myDAQ is reading the
Excitation signal from channel AI0 and Response signal from
AI1.
Slide 24
Set Test Conditions 23 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.
Slide 25
Introduction to Aliasing Set your VI to take 10 samples at a
rate of 10 Samples/s. The output should look similar to this:
24
Slide 26
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.
Slide 27
Grounding the myDAQ Some computers have grounding issues when
using the myDAQs to measure a voltage, including older Fujitsu
models. This causes noisy looking signals, such as that seen
below:
Slide 28
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
Slide 29
Grounding the myDAQ Power Supply GND
Slide 30
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). 29
Slide 31
Increased Sampling Rate 30
Slide 32
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 come back to this later. 31
Slide 33
Front Panel at Natural Frequency 32 Phase difference of 90
degrees Vertical ellipse.
Slide 34
Turn Off Equipment Shut off all of the equipment to prepare for
the next phase of todays lab. 33
Slide 35
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. 34
Slide 36
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 35
Slide 37
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. 36
Slide 38
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. 37
Slide 39
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.) 38
Slide 40
Configuring the DAQ for Analog Output From the Block Diagram of
your VI, Right click and select Input, DAQ Assistant. 39
Slide 41
Place VI on the Diagram 40
Slide 42
Configure the DAQ Assistant Select generate signals, this is
the output portion of the DAQ 41
Slide 43
Configure the DAQ Assistant Select Analog output 42
Slide 44
Configure the DAQ Assistant Select Voltage 43
Slide 45
Select physical output channel Select an output port 44
Slide 46
Configure the DAQ Assistant Select 1 Sample from the generation
mode, finished! If the window below does not show up, change the
window size and the window should appear! 45
Slide 47
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 46
Slide 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. 47
Slide 49
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 48
Slide 50
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. 49
Slide 51
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 to BNC cable. 50 BNC to BNC connector From myDAQ
Slide 52
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). 51 From function generator VCF
input Power supply to proximeter connection
Slide 53
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 52
Slide 54
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. 53
Slide 55
Calibrations 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 function generator. 54 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.
Slide 56
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. 55 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.
Slide 57
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. 56
Slide 58
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 57
Slide 59
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. 58
Slide 60
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 beams natural frequency. See how much closer
to a phase of 90 degrees you can get, compared to manually
adjusting the frequency knob. 59
Slide 61
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. 60
Slide 62
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 61
Slide 63
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 62
Slide 64
myDAQ Resolution 63
Slide 65
Download Code Download the OutputSineWave.vi from the Scholar
site, under Resources-> Instrumentation->Lab 3 64
Slide 66
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. 65
Slide 67
Device Connections 66
Slide 68
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. 67
Slide 69
Oscilloscope Display 68 Function Generator Signal myDAQ
Signal
Slide 70
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), rather than to replace it. 69
Slide 71
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
Slide 72
Modify Homework/Lab 3 Code Modify todays 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
Slide 73
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).
72
Slide 74
Final Modifications 73
Slide 75
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, well have to learn about for
loops and LabView data storage through arrays. 74
