Some questions about strain measurement for DAQ970A

Frequently Asked Questions (FAQs)

Summary

How to use DAQ970A measure strain

Question

Answer

Q1：Bridge configurations test resistances or voltage? Which test function？

A1: Ultimately, bridge configurations is to measure strain, so in the DAQ970A you should choose Strain as the measurement type [Measure > Strain]. And for the setup, you will need to connect strain gages with the selected bridge configurations. Strain gages convert tension, force, pressure, etc into resistance, but the instrument is able to do the conversion of resistance to voltage (strain). The calculation is built-in.

Q2: For Strain measurement，there is many information that can be referred to, well there are eight different configuration test strain, how should I choose in different situation?

Full and half bridge in bending arrangement

Full and half bridge in Poisson arrangement

Full bridge in bending Poisson arrangement

Quarter bridge arrangement

2-wire thermistor

4-wire thermistor

A2: As for the strain measurement, ultimately, strain measurement using bridge configurations are more accurate than direct resistance measurements. It is also able to measure very low resistances.

Direct resistance method:

Between 2-wire and 4-wire, 4-wire method can measure lower resistances as compared to the 2-wire method.

Bridge configuration:

Between the methods in bridge configuration, they are normally categorized into quarter-bridge, half-bridge or full-bridge. The sensitivity of the measurements increases from quarter, half to bridge. So, full bridge provides the highest sensitivity and the fewest error components as there are four active strain gauges connected. Also, the full bridge produces the highest output, so noise is a less significant factor in the measurement. Full bridge is the recommended settings when possible. Full bridge or half bridge circuits should be used in stress analysis if different kinds of interference need to be eliminated.

Quarter bridge is only recommended for measurements under stable temperatures or if temperature compensated strain gages are used together with the arrangement. Also, pure quarter bridge circuits have high sensitivity to interference, so they should only be used under the most favorable conditions or if all interference effects are well known and can be compensated appropriately.

Q3: The Poisson effect is a fundamental physical phenomenon in material mechanics, that is, when an object is subjected to compressive or tensile loads in one direction, expansion or contraction deformation will occur in the other two perpendicular directions.

Well for different Bridge configurations，how to explanation the meaning and difference along different arrangement( bending arrangement, bending Poisson arrangement, bridge arrangement)

A3: The arrangement really depends on the preference of the user, on how much the coverage the user wants. In the image below, I drew a simple illustration on the three different arrangements.

A bending arrangement is simply two strain gages in parallel on two planes, Plane A and Plane B.

A Poisson is whereby the second strain gage is perpendicular to the first gage, but on the same plane.

A bending Poisson is a combination of bending and Poisson. This arrangement is able to combine the tension measured at multiple points with different orientation. So, in other words, there would be more measurements captured, and user will be able to see more sets of data.

The arrangement does not really concern accuracy, it's more to the preference of user on what data is he looking for.

more details refer to below linkage of blog:

https://blogs.keysight.com/blogs/tech/bench.entry.html/2019/07/25/how_to_measure_strai-ZqpP.html