Want to use Simcenter Testlab (formerly LMS Test.Lab) to measure strain gauges? This article explain how for both Simcenter Testlab and Simcenter Testlab Neo using Simcenter SCADAS hardware.
Strain gauges can be hooked up directly to the Simcenter SCADAS frontend. The SCADAS will provide all the needed signal conditioning.
Supported scenarios include:
Direct connection between gauge and SCADAS
Connection thru a slip ring to a strain gauge installed on a rotating part
2. VB8-II and VB8-III Cards
First, you will need a VB8-II or VB8-III (referred to as VB8 for remainder of article) card in your SCADAS Mobile or SCADAS Lab. The VB8 card has strain gauge conditioning capabilities: supply voltages, completion resistors, shunt resistors, sense line support, etc.
A VB8 card also support other types of transducers in addition to strain gauges: potentiometers, ICP/IEPE devices, as well as voltage inputs.
The cards (Figure 1) have 8 channels with software selectable signal conditioning, which can be set independently per channel. Users can select from: ICP, Voltage, Bridges (Quarter, Half, Full), Potentiometer, and Active Sensors.
Figure 1: VB8-II card and with Open Wire (other end is LEMO) cable and BNC wire (other end is LEMO) cable
The card can be installed in any SCADAS Mobile or SCADAS Lab frame. The card can be mixed with other signal condition cards (like V-24, V8-E, etc).
3. Wiring
The VB8-II and VB8-III card has 7 pin LEMO-FGB.08.307 connections to accommodate power supplies, signal wires, ICP/IEPE and sense lines. The card comes with two sets of cables to accommodate BNC and any other types of connectors: a LEMO to BNC pigtail cables, and a LEMO to open wire. Wiring diagram for the card inputs is shown in Figure 2.
Figure 2: LEMO-FGB.08.307 Pinout for VB8-II card
There are 16 cables total delivered with each card, 8 cables of each type: 7-pin LEMO to BNC cable (for ICP/IEPE and Voltage inputs) and 7-pin LEMO to Open Wire (for strain gauges, bridges, active sensors, etc.).
It is necessary to wire the strain gauge to the open wire provided with the VB8 card. The most common connections to make are shown in Figure 3:
Figure 3: Common strain gauge wiring connections for VB8-II card
Quarter, half and full bridges are all supported. Generally, when using strain gauges, the more wires, the better the quality of the signal. In a full gauge, the signal is carried on two wires (a differential input), allowing common mode rejection to be employed to reject noise and Electro-Magnetic Field (EMF) interference.
On quarter and half bridges, the signal is only carried on single wire (a single-ended input) which makes common mode rejection not possible, creating more susceptibility to EMF. Strain gauges, with their long wires and low voltage levels are particularly susceptible to EMF.
In Simcenter Testlab Signature Channel Setup worksheet, set the following to use the strain gauge (Note: If some fields are not showing, make them visible under “Tools -> Channel Setup Visibility”):
Input Mode: Select Quarter, Half, or Full Bridge as desired. Note that AC or DC coupling is also part of this selection. If in doubt, select DC.
Figure 4: Simcenter Testlab pulldown menu choices for input mode
Bridge gage resistance: This is a completion resistor used for quarter bridges, usually either 350 or 120 ohms. It can be set independently per channel. Check your gauge calibration sheet for the correct value.
Bridge strain gage factor: Usually a value around “2”. Check your gauge calibration sheet for the correct value.
Offset zeroing: Possible values are Always/Never/Once, default is 'Always'. Channels marked as 'Always' will be have an offset applied to make the signal read zero when performing a Zeroing operation. Zeroing of the channel is done in the Calibration or Measure worksheet, and is covered in-depth later in this article. If a channel should not be zeroed, then set to 'Never' (for example, a non-gauge transducer). The mode 'Once' is not often used - Zeroing can only be done in the Acquisition Setup screen by switching from Autorange mode to Zeroing at the bottom of the screen. The 'Once' mode was intended to be used if a gauge had to be initially set to zero white setting up the test, before the measurement campaign started. It prevents the gauge from accidentally being zeroed again in the Measurement worksheet.
Actual Sensitivity: The mV/V/EU (Engineering Unit) value. This value can be calculated during calibration or can be entered directly from the specification sheet if it is known.
Simulated Value: If using a strain gauge to measure a value other than strain (for example, could be load/force for strain gauge based load cell), this is where the expected value can be set for a 100 kOhm shunt resistor in units other than strain.
Range: To avoid quantization errors the range should be set to 0.1 V for strain gauges. The default for strain gauges is 0.1 V, not 10 V. This is because strain gauge signals are very low voltage levels compared to other transducers.
4.2 Virtual Channels
In the upper right corner of the Channel Setup worksheet, "Virtual Channels" can be selected under the Channel Setup pulldown:
Math channels (using operators +, -, *, /) can be calculated from the strain measurements.
This section can be skipped if the all the needed strain gauge calibration information is already provided. Go to the section titled “Software: Calibration Verification” to validate the supplied information is correct.
Otherwise, proceed with this section to perform a strain gauge calibration directly in Simcenter Testlab.
The sensitivity value of the gauge can be calculated in the Simcenter Testlab software via the “Calibration” worksheet (Figure 6).
Figure 6: Calibration Worksheet
Click on the “Calibration” worksheet. Once the worksheet is opened, click in the upper right corner, and select “Bridge Settings” (default is AC Calibration) as shown in Figure 7:
Figure 7: Select "Bridge Settings" in the upper right hand corner of the Calibration worksheet
After selecting the channels to be calibrated, do the following:
Press the “Perform Bridge Nulling” button
Select “Perform Calibration” button
Press “Accept” when calibration finishes.
The system calibrates the gauge with two data points: a zero and at a shunt value. The “Perform Bridge Nulling” zeros the gauge.
If there are problems with the gauge (for example, excessive drift due to temperature, or an improper wiring installation) usually the nulling or calibration will fail. A big red message “Error during Calibration” will appear at the bottom of the screen as shown in Figure 8:
Figure 8: If the gauge is improperly wired or not connected, an error occurs during calibration.
The cause of failure can be in the ShuntCalibrationStatus field. For example, if the gauge is installed improperly and is electronically drifting, the message “Unstable Offset” may be displayed as shown in Figure 9:
Figure 9: The type of error encountered during calibration is indicated in the ShuntCalibrationStatus field.
After correcting the problem with the gauge or gauge setup, the calibration should proceed without error. In the lower right corner of the Calibration worksheet, the "Advanced..." button has the criteria used to determine if the gauge has a stable signal. The default settings check if the signal has low variation for at least 3 seconds to determine if the gauge is working properly and in a stable manner.
If successful, a big green message at the bottom of the screen is shown which says “Done – Press ‘Accept’ to save the results (Figure 10).
Figure 10: The status bar at the bottom of the screen turns green when the calibration is successful.
After pressing the “Accept” button in the lower right corner, one can proceed to the “Measure” worksheet.
4.4 Calibration Verification
To check that the strain gauges are working properly at any time, a “Shunt Check” can be performed. In a Shunt Check, a known resistance is applied across the gauge and compared to the expected value.
Simcenter SCADAS VB8 cards contain internal shunt resistors that can be applied to the gauges to perform a shunt check. The default shunt resistor is a 100 kOhm shunt.
To apply the shunt resistor, go to the Measure worksheet:
Press “F3 Ranges” tab
Press the “F12 Shunt” tab
Press “Start”. Everything should come back Green.
Press “Stop” when finished.
Figure 11: Success shunt check will have a green status on all channels
If one of the gauges is not working properly, or the structure undergoing test was damaged significantly, the shunt check on that channel may fail as shown in Figure 12:
Picture 12: Channel value will be red if there is a problem during shunt
The channel will be colored red indicating a problem with the gauge setup, the gauge itself, or excessive damage to the part undergoing test.
4.5 Zeroing
To Zero the gauges before measuring, go to the Measure worksheet:
Press “F3 Ranges” tab
Press the “F11 Zero” tab
Press “Start Zero”. Everything should come back Green.
Press “Stop Zero” and “Set Offsets” when finished.
Figure 13: Zeroing menu
It is possible to set which channels are zeroed, and which channels are NOT to be zeroed. Use the “Offset Zeroing” field in “Channel Setup” worksheet and select Always, Once or Never as desired.
4.6 Measure
Go to the “Measure” worksheet (or press F8 if already in the worksheet) to acquire strain gauge data by pressing the Arm button and then the Start button (with Arrow symbol).
Figure 14: Measurement in progress
A useful feature is the “Shunt Measure Sequence”. This will automatically acquire separate 3 second measurements of zero values and shunt value before and after each measurement.
Under the “More…” button on the middle right side of the “Measure” worksheet, turn on the “Automatically Accept Measurement” and “Shunt measure sequence”. Then press the “Close” button.
Figure 15: Options under "More..." button for setting up Shunt measurement sequence
Now when measuring, a blue status “Zeroing” and “Shunting” will appear immediately before and after each measurement. Each time this occurs, a separate three second recording is made.
Figure 16: Shunt measurement sequence automatically takes zero and shunt measurements (default 3 seconds each) before and after acquisition. Status is indicated by blue message.
These separate recordings can be referenced to discover when and if a part yielded in the middle of a measurement campaign. This can be done by comparing before and after shunt and zero values. The before and after shunt measurements are stored as shown in Figure 17:
Figure 17: Time histories of before and after measurements are stored automatically and separately
Turn on the SCADAS system and then navigate to the “Time Data Acquisition” program located in the Testlab Neo Durability folder in the Simcenter Testlab folder collection (Figure 18).
Figure 18: Locating "Time Data Acquisition" in Testlab Neo Durability folder
After opening the program, select the channels tab on the Simcenter Testlab Time Acquisition home screen (Figure 19).
Figure 19: Select "Channels" (upper left) from Simcenter Testlab Neo Time Data Acquisition splash screen.
5.2 Channels View
A table of channels will appear. In the table, each row corresponds to one input channel (Figure 20).
Figure 20: Channel Setup of Simcenter Testlab Neo.
A list of available channels should appear with different editable properties. Under the column listed “Supply”, select the appropriate voltage supply for the strain gauge. Check that the coupling property is set to “DC”, as this is the type of power that strain gauges will need to function. If the channel is plugged in but does not have power supplied to it, it will appear with a red X icon next to it. On the conditioning tab, select the type of strain gauge conditioning by clicking on the drop-down menu (Figure 21).
Figure 21: Channel Setup with Properties tab.
Properties for each channel can be edited in two ways:
If a cell within a channel row is selected, a properties tab will appear and allow a user to edit all properties on the channel (see figure ##).
Otherwise, individual cells within a row can be edited and properties can be dragged into the channel matrix to add columns for editable properties.
To see a specific list of properties useful to strain gauges, Simcenter Testlab Neo offers different views. Select “Bridge View” (Figure 22) to see a curated list of properties, including conditioning, gauge resistance, supply and offset.
Figure 22: "Bridge View" for channel settings in Simcenter Testlab Neo.
5.3 Calibration and Zeroing/Balancing
After the channels have been properly setup, click the calibration tab to perform any of the following:
Calibrate the strain gauge with a shunt resistor.
Zero or balance the gauges.
5.3.1 Balancing/Zeroing
It is important to zero the strain gauges as most strain gauges will have an initial strain due to being installed on a test unit. In Simcenter Testlab Neo, this is called “Balancing” or “Zeroing”. Then navigate to the “Offset Calibration” tab to begin the process of balancing the electrical strain gauge offset as shown in Figure 23.
Figure 23: The Calibration tab has Balancing/Zeroing functionality.
Select a channel to balance by clicking the checkbox on the Balance tab. Once the channel is selected, press the “Balance” button in the lower left corner. This will balance the wheatstone bridge and output the electrical offset. Once the results have been displayed in the “Electrical Offset” tab, select the “Apply” button to apply the settings to the channel (Figure 24).
Figure 24: Electrical offset successfully applied to a channel after balancing.
This process can be repeated to zero a sensor by selecting the “Zeroing” view next to the located next to the “Balancing” view and clicking “Run”.
5.3.2 Shunt Calibration
If needed, begin the shunt calibration process. This is used to calibrate the lead wire of strain gauges by applying a known shunt resistor across the bridge. The lead wire calibration accounts for the increased resistance of the cable running from the DAQ to the strain gauge. For all bridge-based sensors, it is recommended to run this calibration to compensate for the voltage drop from the cable. Select the “Shunt Calibration” located next to the Offset Calibration tab. A new setup view should appear as shown in Figure 25.
Figure 25: Shunt Calibration is available in Calibration tab of Simcenter Testlab Neo.
Click “Balance” to begin the process. Once the calibration is done, select “Apply”. The sensor should now be calibrated and is ready for measurement.
5.4 Measure
Select the “Measurement” tab to begin measuring. Arm the system by selecting “Arm”. This will allow the sensor to begin sending signals to the system and a live data stream will be available to view as shown in Figure 26.
Figure 26: Measure tab of Simcenter Testlab Neo Time Data Acquisition.
Change the run name in either the run name location or the properties tab located on the right side. The measurement settings such as number of averages and run time can also be altered in the run settings properties tab.
Press Start button (lower left) to begin testing. As a strain is applied to the unit under test, the strain gauge will respond and data should be able to be viewed both live and after it has been saved as shown in Figure 27.
Figure 27: Live measurement in Measure tab of Simcenter Testlab Neo Time Data Acquisition.
An offset check or shunt check (both in lower left tabs) can be performed any time during measurement.
