Would a piezo disk from a Motorola tweeter make a good strain guage?

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kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
I have plantar fasciitis and a heel spur. My present day sneakers, bought at Xmas, are already running down but surprisingly, a memory foam insert bought at the Dollar Store is improving things immensely.

I still am going to buy new sneakers, (they call them running shoes these days), and still put in the memory foam insert because with my condition I want the absolute softest ride for the foot.

Adidas has just come out with a new material called Energy Boost that is both supposed to return more energy and be more comfortable than the traditional EVA foam used in sneakers. It's only a little more expensive than what I've been buying. https://www.youtube.com/watch?v=MZnAeK9l5oU

Before I make the switch from the Asics to the Adidas however, I thought it might be both a good idea and some fun to tape some kind of gauge to my heel and try on the sneakers in the store to see which gives the least shock.

I wonder if took apart a motorola horn tweeter, (does Radio Shack still have them?), soldered some wires and connected a multimeter to them if that would fill the bill.

The thing wouldn't have to be hugely accurate, just give some kind of proportional reading so I could tell which shoe transmitted the most shock to the heel.

I really don't care if I look peculiar in the shoe store, in fact I prefer it if the thing works. :)

If not, does anybody have any idea where I could get a strain gauge or other device which would work?
 
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I'd guess the answer is yes. I don't know details of a Motorola piezo; a cheap piezo disc may actually be better for this application.
Your general idea is reasonable, though. Just connecting the wires to a multimeter probably won't work very well. The signal will be just a transient spike that a digital meter will have difficulty locking in on. An op amp boost and some type of hold circuit would seem necessary. Does your meter have a peak hold function? If so, that still may or may not be enough - you'd have to check its specs.
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Will check specs tomorrow, digital meter is downstairs now.

Not sure, think I have an old analog meter around. The reading doesn't have to be exact-I think I can judge approximately a sudden rise and fall on a dial. It would only be the comparison between brands that would be important, not exact data readings. Also, might find one of those inexpensive audio transformers useful in this application.
 
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The amplification may not be necessary, but a hold circuit would surely help. The outputs may not be different enough to differentiate via a rapidly moving meter pointer. A hold circuit might be as simple as a capacitor and some support components, but someone smarter than I would have to step in with practical details.
 
I have plantar fasciitis and a heel spur.

I had the same issue. It was caused by falling arches. I inserted an arch support, I think the name was powersole..and a dr sholls foam insert about 1/4 inch thick.

No problem since.

I suspect plantar fascitis is overused as a diagnosis. Give the supports a try, 20-30 bucks.

Ah, forgot...It happened to my right foot about 10 years ago. I mucked around for almost a year with the intense pain until I figured it out. About 2 years ago, the left foot started hurting, I repeated the fix and it worked.

jn
 
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The piezo tweeter thingies might be too brittle. But there is flexible plastic foil available that is meant to be used for sensors, musical instrument pickups etc. Can't remember who made it.
And as display you might use something like that:

Testing DS0201 Mini Oscilloscope V1.5 - YouTube

It wouls also allow to see the shape of the pulse not only its amplitude.

Regards

Charles
 
A small disc for under the heel should have force distributed evenly enough to avoid breakage. I'd try just a diode followed by any low-leakage capacitor of about 0.1μF or less. Add a 100Ω resistor and momentary pushbutton as a reset, and see if a DMM provides a reading. Since you're just looking at comparison and not accuracy something simple like this might work.
 
2 answers:

1) I also had an incredibly long heel spur, very painful.
Took 6 months to be bearable and 1 year to dissappear, involving a process of de-calcification.
I made my own custom inserts , X-Rays in hand, and glueing many EVA spongy rubber sheet cutouts where oval holes of progressive diameter were cut, so final shape was an inverted cone which applied even pressure *around* the spike.
It worked very well.
Made it bearable and lack of localized pressure (due to my excessive overweight) signaled to my body that it didn't need to keep reinforcing the "house foundations". Oh well.

2) I have done something similar, but one proper way is:
a) Piezo disk : I used "brass" clock alarm disks, you can use Tweeter disks (which are "pure ceramic= brittle) if you sandwich them between 2 aluminum disks or similar, previously applying a drop of silicone caulking material on each side, so there is not *air* anywhere between layers.
You connect it to:

b) a 10M impedance preamp or buffer.
Maybe a FET or a TL072 with a 10M input resistor.
which drives:

c) some kind of rectifier and crude sample and hold , as 100K series resistor, 1uF capacitor, a 470K to 1 M discharge resistor.
Why "crude is better"?
Because it's not a Lab instrument to measure Piezo characteristics or whatever, no need to get a peak voltage and hold it steady for future measurement, I connected it to a squarewave oscillator which got modulated by the detected voltage, so I want detected voltage to be able to go up and down.
If you want to call it a "long time constant average rectifier", fine with me.

What did I build it for?
For fun, of course
I glued the transducer under a wooden staircase as a "people detector", it was fun to hear the "scuiiic scuiiic scuiiic" made by anybody going up and down.
By the way, up and down can be easily distinguished and after sometime we could even relate noise signature to different people.

And *nobody* could go up or down undetected. ;)

In a nutshell, rather than a multimeter, which will show little if anything at all, something like what I described, installed inside a shoe can give you a believable aural clue as to how it dampens your walking.

Just imagine the shop attendant's faces ;)
 
The piezo disk in this application would take a bit of fiddling to work. You'd likely not have great luck with a large diameter part like that used in tweeter drivers. However, the 10 or 12mm parts from PUI Audio, in stock at Digikey, might be workable at making some usable electric output from the direct distortion forces presented to it with some added brass shim material, maybe <=.5mm or so epoxied directly to the original brass disk. Not enough strength in the disk will inevitably lead to fractured piezo material and some level of failure. If you really want to do this, you'd probably be far better off to just get a real accelerometer and go from there.

Sensors, Transducers | Accelerometers | DigiKey
 
Try an accelerometer

You might want to try a one- (or three-) axis accelerometer chip, that is designed to measure G forces. If I recall Analog Devices makes a number of different versions, with various levels of sensitivity. Tie it in with a PIC controller and a memory chip, and you'll have a little item that will log your impacts (and number of steps) and download that info to a PC afterwards. I think there are non-DIYaudio threads on the internet that will guide you in the right direction. I had built (and programmed) one several years ago, to measure G-forces in a racecar I had designed. (Provided me with a lot of interesting data, regarding car suspension and chassis tuning). Good luck.
 
Remember the 80's when 'bio-feedback' was a fad?
Back then I was working in rehabilitation engineering for brain injured children - one idea was to train hemiplegic cerebral palsy individuals to walk efficiently on their affected side with some audible feedback representing the amount of weight they put into their heel as part of the desired heel-toe gait.

The transducer was simply some semi-closed cell foam about 5mm thick with thin copper (or aluminium) foil top and bottom in a shaped, sealed envelope. It was used as a variable capacitor. Capacitance increases with the inverse of the average distance between its plates.

The capacitor was part of a simple RC oscillator (LM555 or similar) feeding some bcd counters etc and set to beep in real time when a certain frequency (proportional to the weight applied) was reached. Within reasonable limits the calibration to weight applied was repeatable and stable.

The process actually worked and my results are published in the Archives of Physical Medicine and Rehabilitation, a peer review medical journal of NY, for what it is worth.
 

nannoo

Member
2013-01-15 4:32 pm
My initial reaction on reading this was to rip a piezo out of one of those greeting cards that records and plays back a message.

To capture the data accurately enough, I'd probably attempt recording the output as an audio stream into a decent DAW software on a laptop, so you can analyse the waveforms created (though there should be simple free analysis software available, the interesting thing to do would be balance a 'step' in each shoe so you the output wave effectively shows you a typical force/time distribution for equal magnitudes of energy transfer)

Johno's suggestion however is probably several leagues ahead of my own.

Alternatively, you could just trust your body to tell you which is more comfortable, ultimately, as an engineer that would be my final test and benchmark, the testing would just be done to achieve the desired result ;)
 
Thanks, (JMFahey) but that is not the point - a piezo device has a dynamic range of micro metres but on a human scale (or foot) is completely mismatched. A foot needs something soft and pliable not rigid and unyielding.

Some of the alternatives we also explored were pressure sensitive conductive polymers, but I am not sure if these still exist. (there was a US source looking for an application at the time and I think these ended up as tactile touch points in robotics). Another of our failed ideas was a fluid filled rubber sack with real strain gauges attached and these were just too soft and would not calibrate.
 
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