I have a resistance based sensor which gives the resistance change when the strain/force is applied. How to measure the unknown resistance change using the Wheatstone bridge circuit.? I mean what are the important things we have to keep in mind while selecting the resistor value.
The initial resistance of the sensor is not the same. it always give the creep resistance value while no force is applied or under the constant load/Force/strain.
Electrical – resistor value in wheatstone bridge
instrumentation-amplifiersignalvoltage measurementwheatstone-bridge
Related Solutions
Wheatstone bridges are mostly used for very small resistance changes in a sensor, like for a strain gage or something like that. Your resistance change is huge, so you don't need one. As most have been saying, put a constant current source into your sensor. In the attached schematic, as the noninverting input of the op amp is at virtual ground, the current going through the sensor will be a constant 1ma. You haven't offered enough of a response profile to allow us to help you to linearize (or if you even need to), but log and inverse log amps can be found here. I have a feeling an inverse log amp will help. Alternatively, you can sample the output (after inverting it once to make it positive), and use a lookup table if that gives you enough sensitivity in your range of interest
simulate this circuit – Schematic created using CircuitLab
You may be seeing thermal EMFs, as well as thermal drift of the resistors.
Blowing hot air with a hot air gun is not the way to test a circuit like that. Use a proper environmental chamber with a fan inside and control the air flow over the board (maybe with a bit of foam). You may have to wait an hour or two for the temperature to stabilize. The hot air gun will lead to wrong conclusions. You may see dynamic changes that even out as the gradients disappear.
Do not have anything on the board that dissipates a lot of power (I assume you are doing this).
Leaded resistors can be a bit less prone to change from external mechanical forces. Try flexing the board a bit and see if your reading changes. Chances are this is not your problem, but it's worth a check.
Related Topic
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Best Answer
A Wheatstone bridge is used to measure tiny changes in R using a fixed V which gives a fairly constant I. When R changes 50%, I is no longer constant. So this bridge is no longer linear or balanced with a wide spread.
This "Wheatstone bridge" does not pass the criteria of a small change so it is not linear as Wheatstone intended when balanced. Normally Rg changes are small in such a bridge which makes this configuration useful.
A more complex ratiometric bridge is needed to make it more linear, if that is what is desired.
But since Force is inverse to R what is the actual transfer function you want? positive V for Inverse R or something else?
I would suggest something else like 1/R gain with null offset adjust around 1.5 to 2Mohms. try to get a 10:1 linear range to 100:1 but null adjust will be a problem for an "autozero design solution."
1st Define your specs for Forces vs R and then tolerance or error budget for component variance, temperature, aging etc.
Below is a "Wheatstone Bridge"
Note the hysteresis on R is much larger than you think. The datasheet shows a mean of 2M not a peak. So I expect there is large variance from sample to sample for gain and offset of mean.