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FSR - Conductive foam readings Answered

Hello!

I am making a prototype of a force sensitive resistor, and i am using, to old 97%kobber coins, metal sheet and a tiny piece of conductive foam that i got from a tv-repair shop.

The setup is not very good, but its a start, and i am planning the next move for better results...

Now... I want some decent understanding of how the readings work, and why I get these results..

First test, are written with black, and and as you can see it, has the same amount of space between 0-1kg and 1-2kg, but 2-3 and 3-4 changes to a very small space? Why is that?

Second question is, that i have used the x10, to read larger forces. 0-2,5kg and 2,5-5kg has same amount of space. Will the space decrease too, when getting further up the scale too? 
- I know the conductive foam has its limits too, but just to be clear of my readings.

Thanks in advance!

I hope you can see the readings on the picture..!

Discussions

The response curve you are getting is based on the properties of the conductive foam. Think of it like a regular steel spring. It is easier to push or pull at the beginning bit as you reach the limit when it is fully stretched or compressed, the harder it is to stretch or press down any further. The more the spring is compressed against itself, the smaller the changes you see.

I got a few of these quantum tunnelling pills, QTP conductive foam, but never really used them. http://www.ebay.com/itm/271773423586 They may work better than the usual conductive foam for force resistors.

Interesting! How much force do you think 3 cant take before more of less reaching their limit?

I need something that can read higher amounts of pressure. 5kg and up far up the scale.

If a few can take 5 kg without problems its just a question of adding more. but if i need 20 to mesure 5kg it might be a bit too many if i want 10 og 20kg mesurements.

know what to buy for the final product but I want to have a working prototype to go field testing and getting responses back.

Again, this doesnt have to be accurate at all. There can be 2kg indifference but if i can get real readings it would be great.

I'm not really sure what you are trying to build but you might want to look into the way electronic digital or postal scales are built. It comes down to transferring the force applied from the entire scale platform to a little electronic strain gauge. The strain gauge is read electronically and translated to a weight you see on the display. I believe the strain gauges check the resistance of a special alloy wire that changes properties when stretched or pulled. Conductive foam just doesn't cut it when you want accuracy or the range of readings you want.

oh, if you are just experimenting, you could also weigh objects by placing it on top of a filled air chamber like an tire inner tube and then checking the pressure or measuring the displacement of liquid if submerged in a tank.

I didnt think of that! That could be a way to get the results.

I want to read the pressure from 3 points of the foot when standing. I need to collectcollec the data and set a % of the entire pressure on each of the 3 points.

So if you ballance correcte and weigh 70kg, both foot should have 35kg spread out on each of the 3 ballance points.

Use pressure sensors, you will never get good results with foam.

Foam is designed to handle static electricity but not to give different reading with different levels of pressure.

Only thing that foam can do good is to provide a high resistance to eliminate static electricity.

As I said this is a prototype.

I am using sensors for the final product, but they cost a few more dollars than this setup which was free (: Plus 2-3 weeks of shipping.. Right now I just need a simple force sensor to determine which of 3 points to have the highest amount of pressure.

But if these readings could be translated into a rough value i kilograms it would be great for further developments too.

Still foam is not suited for this.

The foam has a fixed resistance and the only changes you see are from the compression which causes a reduction in the surface area.

Main problem is that these foams won't stay soft and flexible, over time they will go flat under pressure and simply stay that way, causing inconsistant readings.

You can get little pressure sensors for under 5 bucks on Ebay, but have you thought of using small piezo speakers for your project?

The create a voltage when pressure is applied but only do so if the pressure changes.

Might be enough to differe between one and 2kg objects.