Pressure Sensing

K

k_shirtcliffe

New Member
Hi all,

I need to measure the pressure applied by someone's foot to a plate - what is the best sensor to acheive this?

Is it true that piezo sounders can be used as transducers, if it is, would they be suitable for this application?

(more detail! The sensor is measuring the amount of force generated by an athlete pushing against one of the plates on a set of starting blocks)

Thankyou again,
Kev
 
M

Mycroft2152

Senior Member
The forum has a SEARCH button on the top of the page.

This topic has been covered many times.

Start with "PIEZO" , "footsteps", or "pressure"

TANSTAAFL

Myc Holmes
 
M

Mycroft2152

Senior Member
If you're just sensing if the runner has left the starting block to start a timer, then the simplest way is to use a lever switch.

Myc
 
D

Dippy

Moderator
I'll second eclectic's point about the fiddlyness. They are small. Have Quantum started making a mesh, I'm sure I saw that somwhere.

Is this a timer or false-start device?
 
W

westaust55

Moderator
Last edited:
D

Dippy

Moderator
Yes, though I'd guess that with clever mounting on a resilient (eg neoprene sponge rubber) pad would give an output as the athlete 'took off' and another on the rebound as the foot left the blocks.
I guess strain gauges will be next... and that's another level of interfacing.
Whatever you do will have to be made tough.
 
hippy

hippy

Ex-Staff (retired)
If it's just false start detection then a plate held off by springs with a number of micro switches behind it should work. For actual pressure measurements it may be possible to apply the same technique ... like bathroom scales but more vertical than horizontal.
 
K

k_shirtcliffe

New Member
Essentially the athlete needs to know how much force he is pushing off with to monitor how well his leg muscles are developing! They are a funny breed.

So a numeric output is needed, I have actually got some of the QTC sheet, I'll give that a try!
 
D

Dippy

Moderator
Go for it.

I just hope it has long term repoducibility. All polymers suffer from fatigue and creep, so the long term would be my major conern.

Now you've given more info, I think you'll have to also consider other types of force sensing - that's if you want consistency and accuracy. No doubt you can prove the idea with QTC. I wonder how many bathroom scales use QTC?

Nevertheless, it's a really interesting project. You making your own Lynx Reactime?
 
R

Rickharris

Senior Member
Dippy said:
Go for it.

I just hope it has long term repoducibility. All polymers suffer from fatigue and creep, so the long term would be my major conern.

Now you've given more info, I think you'll have to also consider other types of force sensing - that's if you want consistency and accuracy. No doubt you can prove the idea with QTC. I wonder how many bathroom scales use QTC?

Nevertheless, it's a really interesting project. You making your own Lynx Reactime?
Click to expand...
The QTC isn't as such, a polymer. It has repeatability but can have a relativly long recovery time from stress.

in this case it shouldn't matter.

I would be tempted to use 3 of the QTC pills behind the foot and take an average from each - they are smaller and recover faster.
 
D

Dippy

Moderator
Isn't it a polymer? It looks like one and it walks like one.

I thought it was a cunning polymer with clever processing with nickel or a nickel based compound??

This is part of an email to me from Peratech technical 2 years ago:-

"We do not have any long term data relating to QTC being under
constant pressure and how accurate it would be - but as you mentioned
in your first email - it would be susceptible to "creep" and we could
not guarantee that it would be reliable."


I don't know what else to say apart from "try it". It's bound to give results but I wouldn't trust it over long periods without regular unloading and recalibration. I think you will have to experiment to see it's limits as you would with any polymer (even if it isn't).

Personally, I think it's a great product. It just needs a bit of flexibility in it's format.

At least you can get the ball rolling with it. And if it does work reliably then fantastic. It may be fine in this app. I certainly hope so.
 
M

Mycroft2152

Senior Member
I think you would be more successful in the long term using a strain guage / load sensor.

They are more expensive, but have a proven track record (ouch) with stabilty and durabilty.

Used in scales, some systems have a 1 part in 50,000 resolution. Remember with a PICAXE (internally) you have a max resolution of 1 part in 1024. Of course stabilty requires a very well regulated power supply.

For a quick and dirty approach, tear a part a digital home scale, you may be able to "circuit bend" it enough to be useful.

Myc
 
R

Rickharris

Senior Member
Dippy said:
Isn't it a polymer? It looks like one and it walks like one.

I thought it was a cunning polymer with clever processing with nickel or a nickel based compound??

This is part of an email to me from Peratech technical 2 years ago:-

"We do not have any long term data relating to QTC being under
constant pressure and how accurate it would be - but as you mentioned
in your first email - it would be susceptible to "creep" and we could
not guarantee that it would be reliable."


I don't know what else to say apart from "try it". It's bound to give results but I wouldn't trust it over long periods without regular unloading and recalibration. I think you will have to experiment to see it's limits as you would with any polymer (even if it isn't).

Personally, I think it's a great product. It just needs a bit of flexibility in it's format.

At least you can get the ball rolling with it. And if it does work reliably then fantastic. It may be fine in this app. I certainly hope so.
Click to expand...

I concede! :) the info I have from the sample I got seems to say it was a rubber compound. BUT Memory being what it is at my age. (post 30)
 
D

Dippy

Moderator
No worries... it ain't a competition. (Though sometimes there seems to be a lot of one-upmanship on this forum as well as mines-bigger-than-yours. Ah 'tis only natural.)
And I know someone will chip in now and say rubber is a polymer :)

Anyway, back to the plot. Give it a go.
 
K

krypton_john

Senior Member
k_shirtcliffe said:
Essentially the athlete needs to know how much force he is pushing off with to monitor how well his leg muscles are developing! They are a funny breed.

So a numeric output is needed, I have actually got some of the QTC sheet, I'll give that a try!
Click to expand...
With regard to the sensor pickup - you wouldn't want to measure the pressure of the shoe on the block as it would be error prone due to placement accuracy. Better to design a special block where the foot plate is a lever onto a pressure sensor. That way no matter where they push the same force will be applied to the sensor.

It would be interesting to see these numbers - the human machine is amazing.
 
R

Rickharris

Senior Member
Dippy said:
No worries... it ain't a competition. (Though sometimes there seems to be a lot of one-upmanship on this forum as well as mines-bigger-than-yours. Ah 'tis only natural.)
And I know someone will chip in now and say rubber is a polymer :)

Anyway, back to the plot. Give it a go.
Click to expand...
Hey now, Woh - How did you know about the size - Your not the source of all those Spam emails I get are you?

:) Never mind!

Chemically Rubber is a polymer (I asked my big brother - he works in Rubber
 
R

Rickharris

Senior Member
krypton_john said:
With regard to the sensor pickup - you wouldn't want to measure the pressure of the shoe on the block as it would be error prone due to placement accuracy. Better to design a special block where the foot plate is a lever onto a pressure sensor. That way no matter where they push the same force will be applied to the sensor.

It would be interesting to see these numbers - the human machine is amazing.
Click to expand...
On the same wave length why not spring a platform and connect the moving element to a pot to measure distance moved. Must be proportional to force on the plate.
 
D

Dippy

Moderator
I know it is, I was being gentle.
Yes, I forward all my Spam to you. Those tablets don't work by the way.

Seriously though, on a starting block ANY movement would be unwanted. Please no panic alerts. It is an interesting project and a lot of work needed to get it right; sensor/damping/sampling etc. And built well and safe too. So, no nuts and bolts from Ebay.
 
K

krypton_john

Senior Member
Rickharris said:
On the same wave length why not spring a platform and connect the moving element to a pot to measure distance moved. Must be proportional to force on the plate.
Click to expand...
Interesting idea - would have to have exactly the right spring to allow only a few mm of travel.

I may have made an error in my post - if the athlete pushes from the top or bottom of the block the lever would of course transmit different forces!
 
D

djmikeys

New Member
I have used a strain gauge as an input for a pic 08m before, it works pretty well. I used a strain gauge out of digital kitchen scales along with a strain gauge amp from rapid.
 
M

Mycroft2152

Senior Member
mshorter said:
I have used a strain gauge as an input for a pic 08m before, it works pretty well. I used a strain gauge out of digital kitchen scales along with a strain gauge amp from rapid.
Click to expand...
BINGO!

Which amp from Rapid did you use?

Any project writeup? Photos, schematics, software to share?

I always wanted to build a model rocket test stand. I started on one once back in the '70s using a strain gauge, a voltage to frequency converter and a bunch of TTL chips to make a 4 digit counter / display. Ouch, a lot of chips.

Oh yes, it was called "Total Input Test Stand"

The acronym raised some eyebrows at the National Rocketry Meet.

:)

Myc
 
premelec

premelec

Senior Member
I have seen a device for alerting to excess injury recovery pressure on feet which used a plastic bladder with fluid coupled to a pressure transducer - this could be a
structure with almost no 'give' which would transmit the starting plate presrure instantly to the pressure sense unit. A PET plastic bottle could be used for the bladder.

I have a variety of 'bathroom scale' type units which use anything from rotating optical code wheels to LVDT [balanced inductor with ferrous slug which moves] dispacement measurement... I guess it all depends on how sensitive you need to get. Putting a piece of iron near an oscillating inductor also can be used as the frequency shifts [I saw a 1930s array of many such oscillators descibed used to study wind profile almost instantaneously].
 
D

djmikeys

New Member
Hi,
sorry I just had a look for the amp and coulfnt find it on rapid, turns out I got it from RS. Basically you need both the strain gauge amp and a pcb kit, below are the 2 links.
Also as it happens I do have some pics from the project, all you have to do is click on the Execution-Technology link and there should be some info there:

Project Link:
http://www.interactiveaudio.co.uk/

Strain gauge:
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=846171

Strain Gauge PCB board:
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=435692

Mike
 
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