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I've attached a strain gauge to a bending arm of a robot, and it varies by about 1.5 ohms from 356 to 357.5 ohms. I place this in a Wheatstone bridge.

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The strain gauge is R3. R1, R2, and R4 are all 378.6 ohms (measured by ohmeter) and I added a resistance in series with R3 to make it range from 380 ohms to 381.5.
I put Vo+ and Vo- as the inputs to a differential amplifier.

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Where R1 ~= R2 ~= 1.2 Kohm and R3 ~= R4 ~= 680 Kohm which gives a gain of 567. Vo ranges from about .003 to .006 volts and Vout ranges from around 1.7 Volts to 3.6 Volts. I want my reading to be as stable as possible for a fixed position of the strain gauge but it ranges by about 20 mV peak to peak when the strain gauge is fixed in place. And it is not a stable oscillation; it will just slowly move around that range, changing by about 2 mV per second.

I am measuring this with a voltmeter, and I have also hooked it up to an Arduino. My op amp is an LM741 with +- 8.2 Volts power supply. My voltage source is an Arduino, applying 5 V across the Wheatstone bridge.

I realize that any tiny change in the resistance or voltage source would be amplified and could cause my problem, but I'm not sure how to isolate and fix it. My ohmmeter is only specific to .1 ohms and my voltmeter is only specific to .001 V, so it's hard to tell what is going wrong.

What I can think to check is: how consistent my voltage source is, how consistent my resistance is, and how consistent my op amp is. What is the best way to do those three things given my testing equipment? Is there something else that might be introducing error that I haven't thought of?

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    \$\begingroup\$ replace the strain gauge with a fixed resistor ... do you still see unstable output? \$\endgroup\$
    – jsotola
    Jun 27, 2019 at 20:16
  • \$\begingroup\$ "I place this in a whetstone bridge." Are you talking about one of these? ;) amazon.com/Sharpening-Adjustable-Whetstone-Sharpener-Stainless/… \$\endgroup\$
    – John D
    Jun 27, 2019 at 20:21
  • \$\begingroup\$ My wheatstone is just resistors on a breadboard. John D. \$\endgroup\$
    – Zyzzyphus
    Jun 27, 2019 at 20:24
  • \$\begingroup\$ I'm not currently in my lab, but thanks for the resistor suggestion jstola. I'll check that in the morning \$\endgroup\$
    – Zyzzyphus
    Jun 27, 2019 at 20:24
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    \$\begingroup\$ The 741 is a poor choice, offset current/voltage drift could be an issue with that much gain. You could also have thermocouple effects, or other thermal drift going on. You don't say what voltage rails you're using for the op-amp, or how stable they are? \$\endgroup\$
    – John D
    Jun 27, 2019 at 20:26

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it ranges by about 20 mV peak to peak when the strain gauge is fixed in place. And it is not a stable oscillation; it will just slowly move around that range, changing by about 2 mV per second.

Referenced to the input that is 20mV/567 = 35uV and 3.5uV. Your bridge has 5/(357.5*2) = 7mA passing through each leg, so a voltage change of 35uV corresponds to a resistance change of 35uV/7mA = 0.005Ω in any bridge resistor or connections to it.

In the comments you say:-

My wheatstone is just resistors on a breadboard

If this is a plug-in breadboard you should know that they are notorious for having poor connections. Your bridge and amplifier circuit should be built on a proper PCB or matrix board with soldered connections, and the sensor wires (if not also soldered) should be connected via a low-resistance plug/socket or screw terminals.

Minimum recommended power supply for the LM741 +-10V, but it will probably still work OK at +-8.2V. More concerning is that it does not have a specification for offset drift! So 3.5uV/second drift may be quite normal while the op amp is warming up. If this is unacceptable then you should use a low drift op amp such as the AD708 (offset voltage drift typically 0.1uV/°C). You might also consider using a rail-to-rail op amp which can operate off a single +5V supply, such as the MCP6V01 (offset voltage drift 0.05uV/°C max).

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