Want to use LMS Test.Lab to measure with strain gauges? Here is how to do it:
Strain gauges can be hooked up directly to the LMS SCADAS frontend. The SCADAS will provide all the needed signal conditioning.
Supported scenarios include:
First, you will need a VB8-II card in your ScadasMobile or ScadasLab. The VB8-II card has strain gauge conditioning capabilities: supply voltages, completion resistors, shunt resistors, sense line support, etc.
A VB8-II card also support other types of transducers in addition to strain gauges: potentiometers, ICP/IEPE devices, as well as voltage inputs.
The cards 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.
The card can be installed in any ScadasMobile or ScadasLab frame. The card can be mixed with other signal condition cards (like V-24, V8-E, etc).
The VB8-II 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.
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-II card. The most common connections to make are:
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.
Software: Channel Setup
In LMS Test.Lab 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”):
Math channels can be calculated from the strain measurements. Channels can be filtered and integrated. Rosette calculations can be performed live. See the 'Rosette Strain Gauge' Knowledge base article for more information.
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 LMS Test.Lab
The sensitivity value of the gauge can be calculated in the LMS Test.Lab software via the “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).
After selecting the channels to be calibrated, do the following:
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.
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.
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.
One should get a big green message at the bottom of the screen saying “Done – Press ‘Accept’ to save the results. After press the “Accept” button in the lower right corner, one can proceed to the “Measure” worksheet.
Software: 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.
LMS SCADAS VB8-II 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:
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.
The channel will be colored red indicating a problem with the gauge setup, the gauge itself, or excessive damage to the part undergoing test.
To Zero the gauges before measuring, go to the Measure worksheet:
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.
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).
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.
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.
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.
Hope these instructions are helpful. Feel free to contact us with questions!
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