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:
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.
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.
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 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:
There are three main gauge properties that determine the thermal behavior of a strain gauge:
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.
Using these terms, some example calculations for different gauge configurations are shown in Figure 3.
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:
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).
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!