8 ms·

Strain gauge made out of PCB

116 points by dr_coffee 1y ago

fake-name 1y ago
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 1y ago
  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 1y ago
  I figured that's why they put it in a sealed chassis in the demo.
- margalabargala 1y ago
  All sensors are thermometers, some measure other things too.
- kragen 1y ago
  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 1y ago
    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 1y ago
      You might be able to do that kind of thing with traces on both sides of the FR4?
knotimpressed 1y ago
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 1y ago
  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-Reloading/dp/B0CGWPJ9MX/ https://www.amazon.com/GRAM-PRES-Precision-Milligram-Reloadi...
  - kragen 1y ago
    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 1y ago
      Precision and accuracy is very expensive there is a reason why high end measurement equipment costs as much as it does.
      - kragen 1y ago
        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 5½ significant figures, US$10 multimeters routinely measure voltage to 4½, 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).
rolph 1y ago
Piezoelectric or Strain Gauge Based Force Transducers? https://www.hbkworld.com/en/knowledge/resource-center/articles/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/C00000145 https://www.kistler.com/US/en/piezo-vs.-strain-gauge/C000001...
amelius 1y ago
> Thinner boards will result in a smaller output voltage swing. For the same weight? I would expect the opposite.
- qwery 1y ago
  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 1y ago
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 1y ago
  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 1y ago
    Ah yes, you're right, my bad.
curtisszmania 1y ago
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116 points by dr_coffee 1y ago

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 1y ago
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 1y ago
I figured that's why they put it in a sealed chassis in the demo.
margalabargala 1y ago
All sensors are thermometers, some measure other things too.
kragen 1y ago
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 1y ago
  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 1y ago
    You might be able to do that kind of thing with traces on both sides of the FR4?

knotimpressed 1y ago

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 1y ago
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-Reloading/dp/B0CGWPJ9MX/ https://www.amazon.com/GRAM-PRES-Precision-Milligram-Reloadi...
- kragen 1y ago
  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 1y ago
    Precision and accuracy is very expensive there is a reason why high end measurement equipment costs as much as it does.
    - kragen 1y ago
      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 5½ significant figures, US$10 multimeters routinely measure voltage to 4½, 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).

rolph 1y ago

Piezoelectric or Strain Gauge Based Force Transducers? https://www.hbkworld.com/en/knowledge/resource-center/articles/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/C00000145 https://www.kistler.com/US/en/piezo-vs.-strain-gauge/C000001...

amelius 1y ago

> Thinner boards will result in a smaller output voltage swing. For the same weight? I would expect the opposite.

qwery 1y ago
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 1y ago

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 1y ago
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 1y ago
  Ah yes, you're right, my bad.

curtisszmania 1y ago

[dead]