Strain Gauges: Selecting an Excitation Voltage

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strain_icon.jpg

 

Strain gauges are “ratiometric” transducers – their signal output is proportional to the supply voltage used to power them.  Strain gauges work with a wide variety of voltage supplies.  Typical supply voltages range from one volt to ten volts.

 

But what is the “best” voltage excitation level to use?

 

When selecting an appropriate voltage supply level, there are two opposing considerations:

  • Higher Voltage – A higher voltage excitation improves the signal to noise ratio of the gauge.
  • Lower Voltage – A lower voltage excitation reduces thermally induced errors in a strain gauge measurement.

So while a high voltage improves a strain gauge measurement from a signal to noise point of view, it can create thermally induced errors. A proper balance is needed to ensure a quality measurement.

 

In this article, the signal levels and thermal errors are discussed further, as well as strategies for selecting the proper voltage.

 

Signal Levels

 

The higher the voltage supplied to the gauge, the higher the signal returned by the gauge during measurement for a given load.

 

This helps avoid issues like electro-magnetic interference.  Strain gauge signals are typically in the microvolt range, and can be easily overwhelmed by electrical noise in their wires.  For example, strain gauge wires (Figure 1) placed near power lines can easily pick up electrically induced signals.

 

strain_wires.pngFigure 1: The long cables of a strain measurement system with low level voltage signals are susceptible to electrical interference from nearby power sources.

By using a higher supply voltage, the strain signals in the wires are stronger, and any electro-magnetic noise will have less of an effect on the signal. Higher supply voltages help even when using differential rather than single ended gauges.

 

Generally speaking, a higher voltage is desirable if no additional issues, like thermal drift or bridge burnout, would be introduced.

 

Thermal Errors

 

Thermal drift is an error in the strain measurement.  It caused by self-generated heat in the strain gauge or measurement system.  This heat causes an apparent change in strain that is not actually due to the deformation of the test object.

 

The higher the voltage supplied to a gauge, the more heat generated by the current running through the wires.  This is similar to how the heating elements inside a toaster work.  Ideally, the heat should be dissipated more quickly than it builds up to avoid thermal issues. 

 

Heat can cause an erroneous strain gauge readings, by:

  • Causing the strain gauge to expand and contract relative to the test object.
  • Changing the resistance of the gauge due to the heat. A gauge normally measures strain using the change in resistance due to the deformation of the test object.
  • Changing the gauge factor sensitivity for both AC and DC strains.

There are three main gauge properties that determine the thermal behavior of a strain gauge:

  • Gage Resistance – The lower the gauge resistance, the more current/power (P=V^2/R) drawn for a given excitation. For example, a 120 Ohm gauge will have worse thermal performance than a 350 Ohm gauge, because it draws more power for a given voltage.
  • Thermal Conductivity – The higher the thermal conductivity (λ) of the structure the gauge is mounted upon, the better heat dissipation. For example, a plastic structure dissipates heat more slowly than a steel structure.
  • Power Density – The power density is the gauge power divided by the area (A) of gauge (the area is the looping wires of a gauge shown in Figure 2). The higher the power density of a strain gauge, the worse the thermal performance.

grid_area.pngFigure 2: The area (sometimes called the “grid”) of the gauge highlighted above.

 These properties can be used to determine the maximum supply voltage whose heat would be dissipated when applied to the gauge.  If the supply voltage is significantly higher than can be dissipated, there is a danger that the gauge will overheat and burn up.

 

Maximum Supply Voltage Calculation

 

Using the “RATY” equation (we made this name up), the maximum supply voltage can be determined from Equation 1.

 

equation.pngEquation 1: Maximum supply voltage for lowest thermal error.

Terms:

  • Vmax = Maximum supply voltage permissible for minimal thermal error.
  • R = Gauge resistance, typically 120 ohm, 350 ohm, or 1000 ohm.
  • A = Area of gauge grid, where resistance wires are looped back and forth. See Figure 2.
  • T = Temperature gradient, change in temperature per unit distance, of the area around strain gauge. Example of a typical value is 0.75°C/mm or 0.75°K/mm (remember 1 degree Celsius = 1 degree Kelvin, but offset by 273.15 degrees).
  • λ = Thermal conductivity of part being tested, expressed in units of W/m*K. Steels have high thermal conductivity (50 W/m*K) while plastics have low thermal conductivity (0.05 W/m*K).

Using these terms, some example calculations for different gauge configurations are shown in Figure 3.

 

table.pngFigure 3: Example maximum permissible voltage for varying gauge configurations.

Strain gauge manufacturers (Vishay, Omega, etc.) typically provide excellent guides that include these values, as well as similar equations for determining the maximum supply voltage. Note that imperfections in the gauge, mistakes in installation, and other factors can make this equation invalid.

 

Gauge Design Considerations

 

Based on the terms in Equation 1, the following can be considered when selecting a gauge and installing it:

  • Gauge Area – Use a gauge with a higher grid area to reduce thermal effects on the strain data. A larger grid area dissipates heat faster (Figure 4).

gauge.pngFigure 4: A larger grid area dissipates heat faster.

  • Gauge Resistance – Use higher resistance gauges (like 350 Ohm rather than 120 Ohm) to reduce thermal effects on the strain data. Usually 350 Ohm gauges are physically larger than 120 Ohm.

 

  • Rosette Gauges – Stacked rosette strain gauges, which take up less area by stacking three gauges on top of each other, create more heat than traditional rosette gauges (Figure 5). Do not use a stacked rosette gauge if it is not needed.

 

stacked_versus_normal_rosette.pngFigure 5: Traditional versus stacked rosette strain gauge configurations.

  • Test Structure – Ideally, gauges would be mounted on test objects in areas with high thermal conductivity, like metals rather than plastics. For example, if mounting gauges on plastics, use the lowest possible voltage excitation, for example one volt or less.

Stabilization

If the environmental temperature is not constantly changing, the main thermal effects are seen immediately after the voltage excitation is applied.  If the gauge and measurement system can be allowed to stabilize for a long period of time, the thermal effects on the strain measurement are minimized (Figure 6).

 

stabilization.pngFigure 6: After applying a voltage to the gauge, the thermally induced strain will stabilize, assuming the temperature of the test enviroment is not changing.

 

The less thermal load, the quicker the gauge will stabilize - so it is always best to select gauges with better thermal properties (higher resistance, greater grid area, etc.) when possible.

 

After the voltage is supplied to the gauge, apparent strain (strain not due to deformation of the test object) is created. The apparent strain will stabilize after some time.  After it is stabilized, the gauge can be zeroed, and measurements taken with minimal thermal errors.

 

This stabilization phenomenon can also be used to determine the proper supply voltage without using the equation.  Slowly increase the supply voltage and monitor the apparent strain.  If the strain is unstable, then the voltage is too high.  The voltage supply can then be reduced until the apparent strain is stable. This way, any imperfections in the actual gauge, which are not reflected in Equation 1, are taken into account.

 

Hope this guide about voltage excitation was useful!

 

Questions?  Contact john.hiatt@siemens.com or download the Simcenter SCADAS Brochure.

 

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