Obtaining Accurate Measurements from Strain Gauge

Question

Background I have a strain gauge that I need to obtain measurements from. The product page indicates that the sensor is a half bridge configuration (one active/one compensating element). The sensor has three wires: White (common), Red (active) and Black (compensating). I have never used a strain gauge before, so I may have some fundamental misconceptions about how it works.

My Understanding of the gauge is that I need to measure the resistance and use the equation: \$strain = (R - R_0)/(G*R_0)^2\$, where \$R\$ is the measured resistance, \$R_0\$ is the gauge resistance, and \$G\$ is the gauge factor. I do this for both leads (active and compensating) and subtract the two to find the actual strain.

What I have Tried Using an Agilent Data Acquisition unit, I have successfully measured the resistance of both leads as ~120\$\ \Omega \$, which is expected, and calculated a strain of essentially zero.

My Problem started when I tested the sensor by heating the it (attached to a metal plate) to 50C. I expected the strain to increase, but was surprised to see that it actually decreased. Now I am questioning my basic understanding of how this sensor works. The change was very small, but distinct from the noise (below image).

Additionally, the noise I am getting from the sensor is less than ideal. I suspect some of it is caused by the furnace I am using, as there is a noticeable increase in noise when I turned it on (below image). Update The increased noise level remains even after de-energizing the furnace, so I now believe the increase is caused by the temperature.

Questions

1. Shouldn't strain increase as the temperature increases?
2. Is this level of noise normal? How can I reduce it?
3. Am I using the strain gauge correctly?

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

The OEM website does not show any useful information. I would call or write to them.

But my guess would be to wire the measurement instrument like this and put a plastic RF cap across the input with shielded twisted pair cable.

There is no indication on sensitivity or polarity.

Answer 2

There is something wrong with the equation \$\text{strain} = (R - R_0)/(G*R_0)^2\$, where G is the gage factor, which is: $$ G= \frac{\Delta R / R}{\Delta \ell / \ell}$$

The denominator is \$ \Delta \ell / \ell\$, which is the definition of strain. Thus, when we take the denominator of your expression, it is \$ \left( \Delta R / \text{strain}\right)^2 \$, and the numerator is simply \$ \Delta R\$, and your units aren't working out. The denominator should not be squared, and the correct relationship is: $$\text{strain} = (R - R_0)/(G*R_0)$$

After you unsquare the denominator, you will see a linear relationship between \$\Delta R\$ and \$G\$. The output of your gage, however, is the result of a voltage division. Assuming the passive resistor is of the same value of the strain gage resistance under zero strain,

$$ V_{\text{out}} = V_{\text{excitation}}\frac{R + \Delta R}{2R + \Delta R}$$

assuming the active element is on the bottom, and the relationship between output and strain will be nonlinear, even if you subtract \$ V_{\text{excitation}}/2 \$. Of course, this approaches linearity if \$ R \gg \Delta R \$.

Other than that, the only way your test makes sense is to show noise and temperature drift in the context of signal size. You can't know if noise and drift are too big without considering what you're trying to measure. Kudos for thoroughness, but show what you're trying to measure so we can assess your signal chain.