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How to measure strain gauges with LMS Test.Lab?

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Siemens Experimenter

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Measuring Strain Gauges with LMS Test.Lab

 

Want to use LMS Test.Lab to measure with strain gauges?  Here is how to do it:

 

Supported Scenarios

 

Strain gauges can be hooked up directly to the LMS 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

 

VB8-II Cards

 

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. 

 

Picture 1: VB8-II card and with Open Wire (other end is LEMO) cable and BNC wire (other end is LEMO) cablePicture 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 ScadasMobile or ScadasLab frame.  The card can be mixed with other signal condition cards (like V-24, V8-E, etc).

 

Wiring

 

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.).

 

Picture 2: LEMO-FGB.08.307 Pinout for VB8-II cardPicture 2: LEMO-FGB.08.307 Pinout for VB8-II card

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:

 

Picture 3: Common strain gauge wiring connections for VB8-II cardPicture 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 (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 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.

Picture 4: LMS Test.Lab pulldown menu choices for input modePicture 4: LMS Test.Lab pulldown menu choices for input mode

  • Measure Quantity: Strain or Force as desired.
  • Bridge supply: Set the voltage supply level; around 5 Volts is ideal. Lower voltages make the signal levels low and susceptible to EMF interference, while high voltage supplies may cause thermal drift. Note that the voltage level is set to zero by default to avoid unintended damage to the gauges.
  • 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. If a channel should not be zeroed, then set to Never.  Zeroing of the channel is done in the Calibration or Measure worksheet.

Picture 5: LMS Test.Lab Channel Setup settingsPicture 5: LMS Test.Lab Channel Setup settings

  • Actual Sensitivity: The mV/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.

 

Virtual Channels

 

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.

 

Software: Calibration

 

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.

 

Picture 6: Calibration WorksheetPicture 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).

 

Picture 7: Select "Bridge Settings" in the upper right hand corner of the Calibration worksheetPicture 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. 

 

Picture 8: If the gauge is improperly wired or not connected, an error occurs during calibration.Picture 8: If the gauge is improperly wired or not connected, an error occurs during calibration.

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.

 

Picture 9: The type of error encountered during calibration is indicated in the ShuntCalibrationStatus field.Picture 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.

 

Picture 10: The status bar at the bottom of the screen turns green when the calibration is successful.Picture 10: The status bar at the bottom of the screen turns green when the calibration is successful.

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:

  • Press “F3 Ranges” tab
  • Press the “F12 Shunt” tab
  • Press “Start”. Everything should come back Green.
  • Press “Stop” when finished.

Picture 11: Success shunt check will have a green status on all channelsPicture 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.

 

Picture 12: Channel value will be red if there is a problem during shuntPicture 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.

 

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.

 

 

Picture 13: Zeroing menuPicture 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.

 

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). 

 

Picture 14: Measurement in progressPicture 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.

 

Picture 15: Options under "More..." button for setting up Shunt measurement sequencePicture 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.

 

Picture 16: Shunt measurement sequence automatically takes zero and shunt measurements (default 3 seconds each) before and after acquisition.  Status is indicated by blue message.Picture 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.

 

Picture 17: Time histories of before and after measurements are stored automatically and separatelyPicture 17: Time histories of before and after measurements are stored automatically and separately

Hope these instructions are helpful.  Feel free to contact us with questions!

 

More Fatigue and Durability links: 

More LMS Test.Lab Acquisition Tips:

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