Data Acquisition with LabVIEW
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Data Acquisition with LabVIEW: Temperature
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Temperature Measurement In LabVIEW
• The steps we will follow are:
– Configure the DAQ device in the Measurement & Automation
Explorer (MAX).
– Open LabVIEW.
– Create the DAQ Assistant on the block diagram window.
– Configure the measurement.
– Create the conversion equation from volts to ºC on the block
diagram window.
– Create the chart indicator on the front panel window.
– Wire the DAQ Assistant, conversion equation, and chart
indicator.
– Save and run the program.
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Temperature Measurement Program
Vout
T( C)
volts
Data Acquisition with LabVIEW: Temperature
0.5 (0.01)T( C)
Vout 0.5
0.01
ºC
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Temperature Sensor
USB DAQ
5V
signal
ground
connected
not
connected
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• Temperature Transducer
– Analog Devices TMP36 Low Voltage, Precision, Centigrade
Temperature Sensor
– Applications: HVAC, automotive, electronics
– Features:
•
•
•
•
•
•
•
•
Temperature span: -40ºC to +125ºC
Accuracy ± 2ºC
Linearity ± 0.5ºC
Temperature coefficient of 10 mV/ºC with 5V supply
Output voltage is 750 mV at 25ºC using a single 5V supply
Output Voltage (volts) for T (ºC): V
0.5 (0.01)T
out
Single-supply operation
High-level, low-impedance output
5 volts
Data Acquisition with LabVIEW: Temperature
Analog
Input 0
Bottom View
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• DAQ Device
– NI USB 6008
– Analog Input Resolution
• 12 bits differential
• 11 bits single-ended
– Maximum Analog Input Sample Rate (single channel)
• 10,000 samples / second
– Analog Input: ± 10V (single-ended)
– Analog Output: 0 to 5V, 12-bit resolution
Green LED
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• Digitizing Resolution
– The NI USB 6008 converts the analog voltage signal to
digital values for the computer.
– A low-resolution converter creates a step-type graph while a
higher-resolution converter creates a more continuous graph.
– The difference can be seen in the graph below.
A digital variable is discrete in amplitude (quantized) and discrete in time (sampled).
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• Digitizing Resolution Error
– Digitizing Resolution Error is the difference between the
measured and actual voltage when using an analog-to-digital
converter (ADC).
– Digitizing Resolution Error = (Input voltage range) / (2N)
where N = the ADC resolution.
– The digitizing resolution error is shown in the table below for
the different input voltage ranges for 12-bit resolution.
12-Bit Resolution
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NI USB 6008
Analog Terminal Assignments
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Digital Terminal Assignments
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NI USB 6008
Signal Descriptions
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Grounded Measurement
Data Acquisition with LabVIEW: Temperature
Floating Measurement
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• Selecting a DAQ Device
– Determine the physical properties that need to be measured
now and in the future.
– Select transducers.
– Determine if any signal conditioning is required
– Determine the allowable analog-to-digital conversion error.
– Determine the sample rate required to accurately capture the
physical properties
– Choose the DAQ device that will meet the requirements
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Data Acquisition with LabVIEW: Temperature
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• Initializing the DAQ
– The first step to using the DAQ is to instruct the operating system to
communicate with it.
– To do this use the Measurement and Automation Explorer (MAX)
– MAX can be opened from LabVIEW by selecting Tools >> Measurement
& Automation Explorer
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• We will be using the NI USB 6008 to make temperature
measurements.
– Plug in the USB 6008 to the computer USB port.
– Open up the Measurement & Automation Explorer (MAX).
– Once MAX has opened, expand the Devices and Interfaces
folder.
– After this, expand the Ni-DAQmx devices folder.
– Note under the NI-DAQmx Devices the NI USB-6008:
“Dev 1”
– See the screen shot on the next slide.
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• Opening the DAQ Assistant
– Open a blank VI in LabVIEW and expose the Block
Diagram.
– Right click anywhere on the block diagram.
– Add the DAQ assistant by opening the functions palette and
selecting measurements I/O.
– Next open DAQmx, then click on DAQ Assistant.
– Drag the DAQ Assistant to the block diagram.
– The DAQ Assistant is a graphical interface for configuring
measurement tasks, channels, and scales.
– See screen shots on the next slides.
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Block Diagram
LabVIEW
Front Panel
Blank Virtual Instrument (VI)
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Blank VI Block Diagram Window
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DAQ Assistant
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• Selecting the Signal
See Following
Screen Shots
– The DAQ assistant opens a new window.
– Expand Acquire Signal, then expand Analog Input.
– Click on the Voltage icon.
• Selecting the Sensor Channel
– The temperature sensor is connected to channel 0, so select
ai0 from the DAQ Assistant.
• Sensor Setup
– Maximum voltage 1.5 (corresponds to 100ºC)
– Minimum voltage 0.5 (corresponds to 0ºC)
– Number of samples: 20
Sample Rate: 2 Hz
• Create Conversion Equation
– Right click on the block diagram. Functions >>
Programming >> Numeric
Vout 0.5
T( C)
0.01
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Set the
number (N)
of samples
in the DAQ
assistant to
20 and the
sampling
rate to 2 Hz
as shown.
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Numeric
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Right Click Here
Choose: Create >> Graph Indicator
T( C)
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Vout 0.5
0.01
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Make Changes to Graph
Change Title
X scale: remove auto-scale and change scale
Y scale: remove auto-scale and change scale
Change Plot Type
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• Code Developed by the DAQ Assistant
– The DAQ Assistant automatically develops some code when
it is configured.
– View the code by right clicking on the DAQ assistant and
choosing Open Front Panel.
– Click convert on the window that appears.
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DAQ Software Design and Flow Control
• We introduced the fundamentals of software design and flow
control and integrated software and hardware for a simple data
acquisition application in the previous exercise.
• We now expand these fundamental concepts by developing an
example that continuously measures temperature. Additional
capabilities include:
– Selection with a Case Structure
– Repetition with a While Loop
– Timing
• The user will be able to choose how long to acquire data, the
interval between samples, and the units.
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Continuous Temperature Measurement
• We will modify the current program:
–
–
–
–
–
–
–
–
–
–
–
Add a Case Structure to the block diagram.
Add a control for acquisition time.
Add a control for the sample interval.
Code the False Case to convert voltage to ºF and the True Case to
convert voltage to ºC.
Add a Boolean Control for choice of units.
Add a While Loop and Stop control.
Add time conversion code.
Add a greater than or equal to function.
Add a wait function.
Wire the nodes.
Save a run the program.
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• Implementing User Preferences
– A key element in software design is to understand the
needs of whoever is going to use the program.
– Here we want the user to be able to change the sample rate,
the number of samples, and the temperature units easily
with a GUI (graphical user interface) when making a series
of measurements.
– To control the sample rate and number of samples, use the
terminals on the DAQ Assistant. Move the cursor to the
first and second terminals on the left side of the DAQ
Assistant. Right click and select Create >> Control
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• Algorithms, Pseudo Code, and Flowcharts
– Sophisticated applications require planning and design. A
good design will make the software and hardware easier to
build initially and easier to modify and maintain in the
future.
– Here we begin the design by developing an algorithm – a
procedure or method for solving a problem or producing a
desired result with a computer. The procedure or method
contains the actions the computer should execute and the
order in which they are executed. The procedure requires
us to decompose the problem into small tasks and to define
the relationships between the tasks.
– When designing an algorithm that requires user input, we
must think of the possible ways the user could interact and
design our code to deal with them.
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– Here the steps in our algorithm written in English, the
pseudo code, might be:
•
•
•
•
•
Open the program.
Run the program.
Read sampling parameters and units from the user inputs.
Acquire a measurement from the temperature transducer signal.
If ºC units are selected
– Convert the units and display temperature in ºC
T( C)
Vout 0.5
0.01
• Else use ºF units
– Convert the units and display temperature in ºF
T( F)
• End the program.
Data Acquisition with LabVIEW: Temperature
Vout 0.5
0.01
9
5
32
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• Flowcharts are often used to
represent programs
graphically.
• Different symbols represent
different operations:
– Ovals represent program
start and termination.
– Diamonds represent
selections.
– Parallelograms represent
input and output.
– Rectangles represent all
processes.
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• Selection with a Case Structure
– Next we will give the user the choice of units using the
LabVIEW Case Structure to control the flow of the
program.
– If the user selects ºC, one case will implement the ºC
conversion formula, or another case will execute the ºF
formula.
– Create space on the block diagram for the Case Structure.
– Insert the Case Structure: Functions >> Programming >>
Structures >> Case Structure
– After we place the Case Structure on the block diagram, the
Run Button arrow is broken to remind us there are
additional steps to programming the Case Structure.
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– The green ? on the left side of the Case Structure is the
Case Structure selector terminal. In the default condition, it
accepts a Boolean (True or False) data type control or
constant as input. The value wired to the selector terminal
determines which one of the Case Structure subdiagrams
executes, allowing us to implement the decision from the
flowchart. If the input is true, the program executes the
code inside the True case.
– Move the green ? Lower on the Case Structure border.
Move the cursor over the selector terminal and right-click.
Use the menu to create a control. Label it F or C? (F) to
inform the user that this control changes the units and F is
the default.
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– The True Case is displayed when we first add the Case
Structure to the block diagram. The True Case implements
the voltage to ºC formula, and the data flows out of the
structure to the chart. When wires cross the border of a
structure such as the Case Structure, they create a tunnel for
the data to flow into or out of the structure.
– We need to switch to the case that implements the ºF
conversion, code the formula, and wire to the tunnel.
– Switch from the True Case to the False Case by clicking on
one of the arrows in the Case Structure selector label.
– Complete the coding.
– The Boolean control default is False.
– Compare the block diagram to the flowchart.
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Broken RUN Button
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Selector Label
Tunnel
Case Structure
Selector Terminal
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Dynamic
Integer
Double-precision floating point
Boolean
Note Data Types
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• Repetition with a While Loop
– So far we have used sequence and selection. We need one
more form of control to build powerful, complex programs
– repetition. There are several repetition structures in
LabVIEW, including a While Loop and a For Loop. Here
we will use the While Loop.
– The While Loop will allow the user to display a series of
values continuously without pushing the Run button each
time or without using the Continuous Run button.
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– The addition of the While
Loop changes the pseudo
code to the following:
• Open the program.
• Run the program.
• Read sampling parameters and
units from the user inputs.
• Acquire a measurement from
the temperature transducer
signal.
• If ºC units are selected, convert
the units and display
temperature in ºC
• Else use ºF units and convert the
units and display temperature in
ºF
• If Stop is true, end the program.
Else return to step 3.
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– The While Loop repeats the code it encloses until the
While-Loop Conditional Terminal receives a Boolean value
of True.
– Note that the data from the previous loop iteration remains
on the graph until it is replaced by the data from the next
iteration.
– We can change the sampling parameters while the program
is running and LabVIEW will read them before acquiring
data in each while-loop iteration.
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To implement repetition in the program, insert a While Loop:
Functions >> Programming >> Structures >> While Loop.
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While Loop
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Right click the While-Loop Conditional Terminal
and create a control to stop the loop.
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While Loop
Iteration Terminal
Data Acquisition with LabVIEW: Temperature
Conditional Terminal
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• Display Data As It Is Acquired
– Rather than waiting for 20 samples to be acquired, converted, and
displayed, we can observe the value of each data point as acquired
and converted.
– Open the DAQ Assistant and change it to acquire one sample on
demand.
– Delete the controls for sample rate and number of samples, as you
do not need them anymore.
– In order to display previous data, we will change the Graph to a
Chart. Right-click on the Graph. Select Replace >> Modern >>
Graph >> Waveform Chart.
– A graph will erase and replot each time the While Loop iterates,
but a Chart will retain the data from previous iterations.
– To clear the Chart for subsequent runs, right-click on the Chart,
choose Data Operations >> Clear Chart.
– Save the Program: Temperature Measurement One Sample on
Demand
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• Explicit Execution Timing
– The While Loop will run as fast as it can and essentially
take all of the computer’s resources from other important
activities. Most times, we don’t need that level of
resources or speed.
– We will slow the While Loop’s iteration frequency by
adding the Time Delay VI (On the Block Diagram:
Functions >> Programming >> Timing >> Time Delay)
– Place the Time Delay VI on the block diagram window, a
dialog box appears prompting the user to set the amount of
time delay. Set it to 0.25 s.
– This type of timing is called explicit execution timing. It
controls how quickly a program executes on the computer
processor. It executes the code in the loop and then
“sleeps” until the wait time has elapsed.
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– Alternatively, we can use a Timed While Loop that
executes an iteration of the loop at the period specified. It
provides multirate timing capabilities, precise timing,
feedback on loop execution, timing characteristics that
change dynamically, or several levels of execution priority.
• Software Control Timing
– We may want to acquire data for a specified time duration.
We can implement code that will automatically stop our
program after a specified amount of time.
– Software timing is not exact. It is not deterministic. The
operating system has priority over our program. It can
interrupt processing at any time. Therefore, when exact
timing is critical, use hardware timing in the DAQ
Assistant or use a real-time operating system.
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