Teach-in #18 on time-dependant properties of inshoe force sensors: What people said...

Dear list members,

This Teach-in interested me because we have used the Interlink FSRs as simple
foot switches for more than 10 years.  Since we use the device as a simple
pressure sensitive switch I've never worried too much about the
characteristics that Chris is investigating.  If anyone is interested the
Interlink devices and a specification sheet are available from

Here are my thoughts on the Teach-in questions:

1. Which sensor is most linear?
>From the information presented it seems that the Flexiforce device is more
linear over the test range but has an apparent spike around 45N.  Both devices
are equally linear over smaller ranges.  The Interlink device shows a
logarithmic curve that is smoother but not as linear.

2. Which sensor exhibits most hysteresis?
It's not possible to tell this for certain from the data presented although
I'd take a guess that the Flexiforce device has the most hysteresis since the
curves show two distinct groups in the first illustration.  This could either
be due to hysteresis in the load/unload sequence or a change in the device
characteristics halfway through the test. The second illustration could
suggest that both devices show roughly equal amounts of hysteresis.

3. What is the cause of the strange loop in the
   middle of the Flexiforce calibration curve?
I'd guess that it's mechanical in origin - possibly due to some component of
the device deforming during the test - see [4] below.

4. Do you think the type of backing material
   affects the results?
The sounds like a leading question ... yes the relative hardness or
squishiness of the backing material will be a factor - see [3] above.

5. Which sensor would you choose for in-shoe gait analysis?
Is "none of the above" an option?  The answer really depends on the source of
the glitch in the Flexiforce data - but for the glitch I would choose the
Flexiforce - however, while the Interlink device is less linear in strict
terms it does have a smoother force/output curve that is quite predictable.
So I'd vote for the Interlink device if I had to make a choice between one of
the other.

Edmund Cramp,
Motion Lab Systems, Inc.
4326 Pine Park Drive, Baton Rouge, LA  70809  USA
+1 225 928-4248 (voice, 2 lines), +1 225 928-0261 (fax)
email eac@motion-labs.com

Dear all bewildered Analyzers, Thanks to Ed for the feedback on this Teach-in. I'm afraid nobody gets the (unspecified) prize this week, because nobody explained the cause of that strange loop in the hysteresis curve: /teach-in/hysteresis So I'll now let you into the secret...  It was actually caused by using a top clamp that was too heavy. This clamp is attached to the load cell on the moving head of the Instron machine, and any accelerations of it modify the force that the load cell records. Since the force sensors were seperated from the clamp by a layer of foam they sensed a slightly different force from the Instron load cell. In our experiment, the accelerations were not inconsiderable because we were sewitching the motion of the head up and down very suddenly. Nevertheless, I think it's an interesting finding, and I wonder whether anyone else has ever observed this sort of thing in Instron testing? I've certainly never heard of it, but my colleague, Panos Tsopelas, in our Civil Engineering department, who correctly diagnosed the problem, says he has seen it quite often in the sort of testing he does with much bigger structures. When we repeated the experiment with a clamp of much lower mass, the loop disappeared: /teach-in/hysteresis/light200.gif The more astute of you may ask why the Interlink sensor did not record the phenomenon, and this leads us onto a further consideration. The Interlink sensor is clearly non-linear. It has, in fact, logarithmic response curve, which attenuates changes at higher forces. If you look closely, you can see a small area of flattening in the Interlink response, especially when the response is plotted on a log scale: /teach-in/hysteresis/log.gif (note that also the quantization from the ADC at the low end of this scale!). So, as Edi Cramp pointed out, the Interlink sensor seems to be less linear, but also has less hysteresis, while the new Flexiforce sensor have good linearity but more hysteresis.  So the question remains: which sensor is best? Chris -- Dr. Chris Kirtley MD PhD Associate Professor HomeCare Technologies for the 21st Century (Whitaker Foundation) NIDRR Rehabilitation Engineering Research Center on TeleRehabilitation Dept. of Biomedical Engineering Catholic University of America

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