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I'd like to measure the force that a human leg impacts on a knee brace during normal gait. I have a problem in finding a suitable sensor for the job...

The leg (and hence the brace) are not flat at their contact surface and it seems FSRs change their response dramatically when bent... and load cells seem quite bulky and also have the same restriction.

I looked at more exotic solutions, e.g. Creating my own sensor out of Velostat (http://www.robotshop.com/uk/pressure-sensitive-conductive-sheet-velostat-linqstat.html) but my concern is it won't be accurate or reliable.

Can strain sensors give me low creep and good repeatability?

Voltage Spike
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John S
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Use a strain gage.

Super-glue a full wheatstone bridge onto the structure. Amplify it by 100 to 1000x and your good to go.

Calibration is easy with a precision shunt resistor (to get strain). Then use some maths to estimate load.

It's a bit pricey for quality gages, and you will still need to acquire the voltages. But once it's in place, it will last a long time.

If you design it as a in-series module, then you could directly calibrate it with dead-weights or a press, and it's even easier (I think that it's still a good idea to have a shunt resistor so you can detect if the gages are drifting or damaged).

Vishay has some good app notes on the subject.

Chris Knudsen
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  • Awesome! Thank you very much for this. Would it make sense to use a set of 4 to have all sides of the bridge active? – John S Jun 28 '17 at 15:09
  • Full bridge gages are available on one substrate. Buy and apply. The stuff we commonly do is laboratory grade, so my sources may be out of budget. For high rel, we use epoxy for the adhesive (non-gassing), and cure at elevated temps. For less-rel, we apply with super glue (and it works fine). Many of the spiffy DAQ systems out there can interface directly to these low-level signals (combine the words lab and jack, and google the results). Tell them I sent you, and maybe they'll send me a cookie. – Chris Knudsen Jun 28 '17 at 21:16
  • Direct your attention to https://electronics.stackexchange.com/questions/312460/measuring-tension-force-on-rope. You might have good suggestions. – Paul Uszak Jun 29 '17 at 12:14