[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.

[kragen]

I hadn't thought about the hygroscopic and expansion questions; I think FR4 is, like wood, almost immune to longitudinal variation with temperature and humidity due to its anisotropy. (But I haven't tried to measure that.) And strain gauges are low enough impedance that I'd expect the capacitive effects to matter.

The temperature coefficient of resistance of the strain element seems like a concern, though, and so do thermal EMFs. My kitchen scale zeroes when I turn it on, a procedure that should be able to cancel one of these two but not both. Maybe you could have a diode thermal sensor, as an MCXO does, to measure the temperature so you can cancel both?

This project seems to do the first-order temperature correction thing:

> The included sample firmware will wait until a serial console is opened, perform a 5 second offset calibration, then sample continuously at the lowest gain setting. The graph.py script can be used to display the output.

> For high sensitivity measurements, it's important to let the board reach a stable operating temperature for at least 5 minutes before calibrating.

The thing I intuitively worry about here is creep. Does FR4 creep enough to worry about? Normally you make strain gauge bodies out of steel because it doesn't.

[4gotunameagain]

For temperature dependent effects there are things like self-compensated strain gauges (where their temperature related deformation counteracts that of a target material), or dual side measurement (sandwich a bar between two strain gauges, strain cause compression on one, elongation on the other, so +-/-+, while temperature compresses or elongates both (++/--), cancels out.)

[kragen]

You might be able to do that kind of thing with traces on both sides of the FR4?

[margalabargala]

All sensors are thermometers, some measure other things too.

[mofosyne]

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.

[greggsy]

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