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Strain Gauge Bridge Completion Modules

The most common strain gauges used to quantify the state of stress on a test specimen’s surface, are uniaxial and rosette gauges.  For accurate measurements of stress and strain, these uniaxial and rosette gauges are independently connected as a Wheatstone bridge in a 3-wire quarter-bridge or half-bridge arrangement.

Today, most high-end data acquisition equipment manufacturers provide signal conditioning options for collecting data from single strain gauges.  Signal conditioning for strain gauges usually includes circuitry for bridge excitation voltage, quarter and half bridge completion arrangements, and shunt calibration.

Sometimes you may need to view or collect data from a single strain gauge using a device that only has analog voltage inputs and no strain signal conditioning.  This was precisely the case when I was working with the HX711 load cell/strain amplifier.  The HX711 requires a full bridge input so I instrumented my test specimens as such.  If I needed to use a single strain gauge with the HX711, I would have had to use an external bridge completion circuit.

What are your options for measuring single strain gauges with a device that only has voltage inputs or full bridge inputs?

Option 1: Buy a commercial off the shelf bridge completion modules.

The list below gives the specifications for some available bridge completion modules.  I plan on adding more completion modules to this list for future reference, so send me any additional completion options.

Strain Gauge Completion Modules
 
Manufacturer NI VPG Campbell Scientific
Model # NI 9926, NI 9945, NI 9944 MR1-10C-129, MR1-350-130, MR1-120-133 4WFBS1K, 4WFBS350, 4WFBS120
Description 3-Wire Quarter Bridge Completion 3-Wire Quarter Bridge Completion 3-Wire Quarter Bridge Completion
Resistance 1000 Ω, 350 Ω, 120 Ω 1000 Ω, 350 Ω, 120 Ω 1000 Ω, 350 Ω, 120 Ω
Strain Gauge Connector Terminal Block Solder Tab Terminal Block
Device Connector RJ50 Solder Tab Pins and Lead Wire

Option 2: Build your own circuit.

If you are building a product or are in the strain business long term, building your own circuit may be a cost effective alternative to the potentially more expensive off the shelf bridge completion option.  I have built a few bridge completion circuits in the past.  Here is a list of things to keep in mind when designing a circuit.

  1. Use high precision, low resistance temperature coefficient resistors
  2. The voltage source used for bridge excitation should be from a stable source like a reference since the output of a Wheatstone bridge is inversely proportional to the excitation voltage Vout/Vex.
  3. Use remote sensing to compensate for errors in excitation voltage from long lead wires
  4. Amplification will increase measurement resolution and improve signal-to-noise ratio
  5. Filter data to remove external, high-frequency noise

For more information about bridge completion or our strain gauging services, contact Ryan Welker @ (844) 837-8797 x701.

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Strain Gauge Shunt Equivalent Calculations in iTestSystem

When making strain measurements it is important to perform a shunt calibration both before and after the actual measurements are acquired.  Shunt calibrations ensure accurate strain measurements by adjusting the sensitivity or gain of the data acquisition equipment to compensate for leadwire resistance and other scaling errors.

iTestSystem takes advantage of the shunt calibration circuits included in the National Instruments (NI) cDAQ strain modules.  The NI-9235, NI-9236, and NI-9237 strain modules contain an internal shunt resistor that when switched on “shunts” across one leg of the strain circuit’s wheatstone bridge.  When active, the shunt resistor offsets the strain measurement by a constant strain which is calculated using the equivalent shunt calculation.  The equivalent strain/shunt value is dependent on the strain gauge configuration, gauge resistance, shunt resistance, gauge factor, and material properties.

In the latest version of iTestSystem, we added a built-in strain gauge shunt equivalent calculator that can be accessed from the strain configuration page.  This calculator has allowed us to speed up the calibration process and eliminate hand calculation errors.

For a free trial of iTestSystem and the equivalent shunt calculation tool, contact chase.petzinger@itestsystem.com.