[HN Gopher] Strain gauge made out of PCB
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Strain gauge made out of PCB
Author : dr_coffee
Score : 97 points
Date : 2025-05-07 13:32 UTC (4 days ago)
(HTM) web link (github.com)
(TXT) w3m dump (github.com)
| fake-name wrote:
| 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.
| mofosyne wrote:
| 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 wrote:
| I figured that's why they put it in a sealed chassis in the
| demo.
| margalabargala wrote:
| All sensors are thermometers, some measure other things too.
| kragen wrote:
| 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.
| knotimpressed wrote:
| 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?
| colechristensen wrote:
| 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...
| kragen wrote:
| Not every scale with milligram repeatability has milligram
| accuracy, and not every scale with milligram resolution
| readouts has milligram repeatability. You probably know that,
| but not everyone reading this does.
|
| (This one might be fine? It does claim to have a 50.000g
| calibration weight, which is a good sign, but it doesn't say
| anything about metrological traceability, which is a bad
| sign.)
| dogma1138 wrote:
| Precision and accuracy is very expensive there is a reason
| why high end measurement equipment costs as much as it
| does.
| kragen wrote:
| At some level it becomes expensive, but it's far from
| clear that five significant figures of mass is that
| level.
|
| A US$2 quartz watch measures time to 51/2 significant
| figures, US$10 multimeters routinely measure voltage to
| 41/2, and US$5 GPS receivers can provide you with time
| measurements accurate to 40ns that inherit the drift of
| world metrology standards, a precision of 16 significant
| figures if you are measuring a long enough time interval
| (over 10 years).
| userbinator wrote:
| Where are you finding $10 4.5 digit multimeters?
| kragen wrote:
| Hardware store, usually. 3.5 digits is common, 4.5 digits
| less so.
| userbinator wrote:
| 3.5 digit ones sell for far less than that (especially
| the infamous 830-series based on the ICL7106 and its
| clones). I haven't found any 4.5 digit ones at that
| price.
| thirdhaf wrote:
| As a quick and dirty rule of thumb measuring parts per
| million in anything except time or frequency will get
| expensive. Temperature drifts will cause expansions and
| contractions on that order if you're measuring lengths.
| kragen wrote:
| Yes, but in this case we're measuring weights, not
| lengths, and we're looking for 5 sig figs, not 6.
| colechristensen wrote:
| >metrological traceability
|
| It's a $20 scale, if you had need of metrology you wouldn't
| be buying a $20 scale. Most of us do not need metrology. I
| want to make small amounts of pickles with perhaps
| unreasonably precise measurements of salt at scales where
| half a gram or maybe a tenth of a gram is significant.
| s0rce wrote:
| 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.
| kragen wrote:
| Thanks for the reference! If you do find the link maybe it
| will be in the WABAC machine.
| stavros wrote:
| That's the most interesting misspelling of "wayback" I've
| seen so far.
| bialpio wrote:
| TIL it may not necessarily be a misspelling: https://en.m
| .wikipedia.org/wiki/Wayback_Machine_(Peabody%27s...
| flowerthoughts wrote:
| 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
| kragen wrote:
| My uninformed opinion based on no experience is that this will
| creep so you should use a Kibble balance instead. (See sibling
| comment by s0rce.) Or maybe use a lever arm to amplify
| milligrams into tens of milligrams of force. Or just a regular
| pan balance. How big a weight do you want milligram precision
| on?
|
| If you have some way to cut a precise shape out of some kind of
| metal sheet of well-controlled thickness, could you cut out a
| milligram precision bismar balance or steelyard?
| dogma1138 wrote:
| Probably not, PCBs are also terrible as load cells because
| fibers break.
|
| In general if you want a precise and accurate strain gauge
| you'll be paying a lot for it, especially for one that doesn't
| need to be recalibrated before every use and after nearly every
| measurement.
| stefan_ wrote:
| Shahriar has a video on repairing a milligram resolution scale:
| https://www.youtube.com/watch?v=Lz9mBc6FGzE
|
| They don't tend to use strain gauges I think.
| klysm wrote:
| Milligram accuracy isn't a really a direct property of the load
| cell. A lot of it comes down to creep and hysteresis behavior.
| rolph wrote:
| Piezoelectric or Strain Gauge Based Force Transducers?
|
| https://www.hbkworld.com/en/knowledge/resource-center/articl...
|
| Piezo vs. strain gauge https://www.kistler.com/US/en/piezo-
| vs.-strain-gauge/C000001...
| amelius wrote:
| > Thinner boards will result in a smaller output voltage swing.
|
| For the same weight? I would expect the opposite.
| qwery wrote:
| For the same deflection, I assume.
|
| The device effectively measures mechanical strain at the
| surface of the PCB. The surfaces of a thicker board will
| experience more strain from bending because the radius of the
| curve (at the surface) is greater.
| amelius wrote:
| Can anyone explain why the BRIDGE_SUPPLY voltage is connected to
| the voltage regulator output and PWM signal at the same time
| (through the FSA5157L6 analog switch)?
| bitdivision wrote:
| I don't think it is. PWM_BIAS is used as the select for the
| analog switch. High connects COM to B1 (GND) and low connects
| COM to B0 (BRIDGE_SUPPLY).
| amelius wrote:
| Ah yes, you're right, my bad.
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