Flexible force sensors for running analytics

Thread Starter

Super_yeti_man

Joined Nov 5, 2019
4
I'm researching possible solutions for measuring the force exerted when running and jumping. The wearable application will be a type of insole and requires flexible sensors capable of reading up to (and likely beyond) 600N without permanently damaging the sensor. Flexible piezoresistive FSRs and capacitive sensors are perfect on form factor, but cannot get anywhere close to this force range from what I can tell. Even solutions currently on the market using these technologies are breaking quite easily out in the field. Is a flexible sensor, or even a smart material from which one could make a sensor, available for this range? Is it even possible?
 

MrSoftware

Joined Oct 29, 2013
1,907
How much precision and what speed (slew rate) do you need? i.e. do you just need to know when a step happens, or do you need to know how much force is in the step to within 1N, 10N, 100N, etc.. ? And do you need to map the force curve precisely? Are the sensors that are failing just typical flexible FSR's, like this (but a different shape)?
 

Thread Starter

Super_yeti_man

Joined Nov 5, 2019
4
How much precision and what speed (slew rate) do you need? i.e. do you just need to know when a step happens, or do you need to know how much force is in the step to within 1N, 10N, 100N, etc.. ? And do you need to map the force curve precisely? Are the sensors that are failing just typical flexible FSR's, like this (but a different shape)?
Thanks for the quick response. Yeah, the FSRs that have been tested are like those, except smaller and circular (adafruit) and these have mostly been used for prototyping purposes. However products such as this have been tested for our context and its been reported that these are also breaking quite often due to an excess in force. (troubling for a final product on the market, however we're looking more at jumping and landing forces rather than running so perhaps that makes sense.) From what I can gather these implement flexible capacitive sensors.

To be honest, precision and slew rate have not been considered as a design constraint at the moment because finding something thin and flexible that can reliably read the forces we're playing with has been tricky enough, but precision to the nearest 10N would be adequate. A precise, repeatable curve would be preferable. Slew rate is not really considered at this time.
 

MrSoftware

Joined Oct 29, 2013
1,907
Unfortunately I'm not aware of anything better than what your link showed, given that their device was designed specifically for shoes. Just food for thought, but maybe wrapping their sensor with some flexible but durable coating would improve durability without affecting the readings too much? Or if one large sensor is breaking, maybe try spreading out multiple smaller sensors. Hopefully someone else will have better info for you.
 

Bernard

Joined Aug 7, 2008
5,648
I bought a sample of " Velostat ", pressure sensitive conductive film from Digi Key, PN 1528-2211-ND, about 1 sq. ft.
There is quite a bit of information on the web.
 
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swr999

Joined Mar 30, 2011
28
Not sure I’d trust this manufacturer, as he has a problem with measurement or geometry... And I quote:
“...with a square, 1.75×1.5", sensing area.”​
Marketing guys probably write this. I wouldn't discount this vendor necessarily based on the difference between a rectangle and a square ;)
 

Thread Starter

Super_yeti_man

Joined Nov 5, 2019
4
Hi, have you considered using the FSRs from tekscan? According to their website they can measure up to 31138 N https://www.tekscan.com/force-sensors

I built a system last week using an A201 and so far it has performed as expected. They also offer custom sensors if it is worth the investment.
I found Tekscan a while ago and I'm not sure why they skipped my mind, but it would seem their sensors would be perfect. They also make sensors specifically for shoes which is promising.

Sorry if this is a basic question. But the website states that "The dynamic range of this small force sensor can be modified by changing the drive voltage and adjusting the resistance of the feedback resistor". What affect does this have on the sensor other than changing the range? Does is affect precision or resolution at all?
 

OBW0549

Joined Mar 2, 2015
3,546
Sorry if this is a basic question. But the website states that "The dynamic range of this small force sensor can be modified by changing the drive voltage and adjusting the resistance of the feedback resistor". What affect does this have on the sensor other than changing the range? Does is affect precision or resolution at all?
I think your assessment is correct; changing drive voltage and/or the feedback resistor simply changes the scale factor.

"Dynamic range" is often incorrectly used as a synonym for range or scale factor, despite the fact that dynamic range is completely different and unrelated.
 

Thread Starter

Super_yeti_man

Joined Nov 5, 2019
4
"Dynamic range" is often incorrectly used as a synonym for range or scale factor, despite the fact that dynamic range is completely different and unrelated.
I think I understand from a quick Googling, but could you please explain the difference? I would greatly appreciate it
 

OBW0549

Joined Mar 2, 2015
3,546
I think I understand from a quick Googling, but could you please explain the difference? I would greatly appreciate it
Sure. Dynamic range is a ratio, usually expressed logarithmically in decibels, between the largest signal a system can handle and the smallest signal that reliably can be distinguished from an absence of signal. Dynamic range is independent on range or scale factor, for example a pressure sensor which operates from zero to 500 psi with a scale factor of 10 mV per psi.

This Wikipedia article does a pretty good job of explaining.
 
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