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Strain gauge made out of PCB

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Re: Strain gauge made out of PCB

#2

Post by fake-name »

The challenge is that while you can make a strain gauge out of just about anything, making them repeatable over temperature, humidity (in the case of hygroscopic materials, like PCB FR4) and repeated flexing is where it gets difficult.

For this, while I'm sure it works, if the humidity and/or temperature changes, the same deflection will result in different readings.

If you can calibrate it immediately before each use, or you don't care about absolute values, this is a completely valid option.

"Real" strain gauges generally use a constantan resistive element to deal with the temperature variability, deposited on a plastic carrier film (typically polyimide). The film elements then get glued to the stress sensing member. They're fairly inexpensive too.

Re: Strain gauge made out of PCB

#4

Post by mofosyne »

fake-name wrote: post #2

The challenge is that while you can make a strain gauge out of just about anything, making them repeatable over temperature, humidity (in the case of hygroscopic materials, like PCB FR4) and repeated flexing is where it gets difficult. For this, while I'm sure it works, if the humidity and/or temperature changes, the same deflection will result in different readings. If you can calibrate it immediately before each us…

I'm sure there's a general rule of thumb where this approach works best as an approximation (such as a simple on/off switch)... which may give us opportunities to simplify the BOM list further with cheaper parts at cost to accuracy which wouldn't apply to PCB anyway.

Where I mostly seen this approach is with 3D printers where we just want to know if the nozzle is touching the print base.

But if we can quantify the general worse case variation between most PCBs then maybe we can create a recommended strain sensor element with a semi-quantified level of accuracy so it's not just an on/off sense.

Re: Strain gauge made out of PCB

#5

Post by colechristensen »

knotimpressed wrote: post #3

For a long time I've been trying to make a DIY milligram-accurate scale, and milligram-accurate strain load cells are expensive. Does anyone know if the resolution of this is high enough?

You can just buy strain gauges which are specially cut copper foils on a thin plastic substrate that you glue to something, like a metal strip. The resistance changes very slightly as the metal bends.

You measure the change in resistance with a wheatstone bridge tuned correctly.

You basically just need a strain gague (a few dollars), 4 resistors, an op-amp, and a microcontroller with an ADC.

Calibration is important and you'll run into things like the metal bar creeping, permanently bending as a result of weight being put on and off.

But also, milligram accurate scales are $20 on Amazon https://www.amazon.com/GRAM-PRES-Precision-Milligram-Reloadi...

Re: Strain gauge made out of PCB

#6

Post by s0rce »

knotimpressed wrote: post #3

For a long time I've been trying to make a DIY milligram-accurate scale, and milligram-accurate strain load cells are expensive. Does anyone know if the resolution of this is high enough?

My dad built one with an electromagnetic coil many years ago, can't find the design, capacity was limited but had 100? microgram resolution, maybe similar to this applied science video https://www.youtube.com/watch?v=ta7nlkI5K5g but I think simpler.

maybe something like this: https://www.erowid.org/archive/rhodium/chemistry/equipment/s... https://www.erowid.org/archive/rhodium/chemistry/equipment/s...

I found some old emails about the scale from 2005/6 but can't find the link and it would probably be gone anyways.

Re: Strain gauge made out of PCB

#8

Post by flowerthoughts »

knotimpressed wrote: post #3

For a long time I've been trying to make a DIY milligram-accurate scale, and milligram-accurate strain load cells are expensive. Does anyone know if the resolution of this is high enough?

In the video, the author tries it with a small component that I think is a SOT-323-5 or similar. Based on [1], that weighs about 5-8 mg.

[1] https://www.mccsemi.com/pdf/ComponentWeightInformation.pdf

Re: Strain gauge made out of PCB

#9

Post by greggsy »

fake-name wrote: post #2

The challenge is that while you can make a strain gauge out of just about anything, making them repeatable over temperature, humidity (in the case of hygroscopic materials, like PCB FR4) and repeated flexing is where it gets difficult. For this, while I'm sure it works, if the humidity and/or temperature changes, the same deflection will result in different readings. If you can calibrate it immediately before each us…

I figured that's why they put it in a sealed chassis in the demo.

Re: Strain gauge made out of PCB

#10

Post by margalabargala »

fake-name wrote: post #2

The challenge is that while you can make a strain gauge out of just about anything, making them repeatable over temperature, humidity (in the case of hygroscopic materials, like PCB FR4) and repeated flexing is where it gets difficult. For this, while I'm sure it works, if the humidity and/or temperature changes, the same deflection will result in different readings. If you can calibrate it immediately before each us…

All sensors are thermometers, some measure other things too.
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