Challenges:
- Custom tests require a variety of sensors/instrumentation
- Large tests can have many sensors, making it difficult to keep track of sensors and data
- Different instrumentation can affect how data is recorded and saved
Values:
- Working with signals acquired from multiple sensors
- Setting up an intuitive virtual instrument panel
- Managing output files
Testing usually requires measuring quantitative data, most commonly acquired by precise and accurate sensors. Additionally, a single test may require multiple types of measurements which would require multiple types of sensors. LabVIEW’s intuitive graphical format enables development of complex, multi-sensor data acquisition for custom-tailored tests.
Example Test Setup
To demonstrate, look at the LabVIEW setup for an optical extensometer. A test article is placed in an Instron machine and will be subjected to a tension, measured by a load cell. The elongation of the test article will be measured by laser distance sensors as they measure the distances to their respective targets that are fixed to the test article. Finally, a thermocouple will measure the ambient air temperature. Note: the scope of this post is limited to working within the LabVIEW software; future posts will cover installing sensors and post-processing test data.
Figure 1: Example test schematic
INside labView
Take a look at the virtual side within LabVIEW. When measuring data using LabVIEW, the first step is to read the signals from the sensors being used with the DAQ Assistant block.
Figure 2: DAQ Assistant block
Initiate the DAQ assistant by identifying what sensors are connected and what signals they provide. Important parameters here that affect both the real-time measuring and the output files are Sampling Rate and Number of Samples. Depending on the test and the sensors implemented, these values should reflect how frequently data points should be measured and recorded. Some sensors have limits on how fast or slow they can be sampled.
Figure 3: DAQ Assistant properties
Unless the intent is to simply write all of the data to an output file, it will be necessary to work with the individual signals by separating the signals with a Split Signals block in LabVIEW. Adjust the size of the block until you have as many output nodes as you do signals. Note, the order here is the same as that configured in the DAQ Assistant.
Figure 4: Merge Signals block
Once split, each signal can be manipulated independently and connected to a different virtual instrument or gauge that appears on the instrument panel.
LabVIEW has many built-in instruments and gauges that give an intuitive indication of what is being measured. For example, rather than just providing a real-time value of the temperature, connect the signal from a thermocouple to a thermometer gauge to create a virtual thermometer that behaves like a real one based on the measured temperature.
Figure 5: Virtual instruments
Without explicit labels or notation, the user can quickly understand what data is being displayed. Similarly, connect the signal from a load cell to a gauge that not only indicates the load value but displays it on a color-coded scale.
Figure 6: Finished block diagram
Figure 7: Finished instrument panel
Output Files
To save the data from a test, it must be written to a file using a Write to Measurement File block in LabVIEW. Combine the signals that are to be recorded into a single signal with a Merge Signals block, and then wire this signal to the Write to Measurement File block.
Figure 8: Merge Signals and Write To Measurement File blocks
When the test is complete and the LabVIEW program stops running, an output file will have been generated. Look at the properties of the Write to Measurement File block and see how these affect the output file.
Figure 9: Write To Measurement File properties
Probably the most influential parameter of these properties is what type of output file will be written. LabVIEW can export directly to an Excel spreadsheet, which makes it easy to work with the data in Excel. However, exporting to Excel is costly in memory and if the test data is large enough, it is possible that Excel would not be able to open the file at all. Instead, the text file options are often preferred for storing large datasets of numeric values.
SUmmary
LabVIEW’s graphical programming consolidates data acquired from a multi-sensor test setup and provides the tools to set up an intuitive virtual instrument panel that makes real-time measurements easy to interpret and can export large datasets from large-scale tests. If you understand the signals coming in and know what needs to be written out, LabVIEW can handle everything in between.
Written by
Robert Nguyen
Robert Nguyen is an Aerospace Engineer with a background in design and manufacturing engineering; he currently works on the manufacturing and testing team. In his free time, Robert enjoys being outdoors, learning different styles of movement, reading, and editing photos and videos.
