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Resistivity. - Conductive packaging foam for IC chips.

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tarakan2

Fri Jan 11 2013, 03:35PM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
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Posts: 179

I am working on an alternative computer input device and I want to make my own force sensors.

I am looking at this datasheet:

I see two values: surface resistivity and a volume resistivity.

How do I find the resistance of a square of this foam, 10mmX10mm, if I place it between two plates on the 10mmX10mm surfaces, each?

Should I pick a high density foam or low density foam for the least amount of residual strain.

Thank you.

Mattski

Sat Jan 12 2013, 06:51AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

You take the volume resistivity in ohm-meters and multiply it by the length over cross-sectional area. Length is along the direction that the current flows, and the cross-sectional area is in the plane perpendicular to this. There's a nice diagram here:

wrote ...
How do I find the resistance of a square of this foam, 10mmX10mm, if I place it between two plates on the 10mmX10mm surfaces, each

In your example, the length would be the thickness you choose of the sheet, and the area be the the 10mm x 10mm area of the foam.

You can ignore the surface resistivity (aka sheet resistivity) values that they provide in the datasheet, that will depend on the thickness of the sheet so they are just providing a range of values.

tarakan2

Sun Jan 13 2013, 01:32PM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
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Posts: 179

So 35 * (0.006/0.0001) = 21,000 resistance in ohms of a 10mm^2 cell

where

0.0001 Meters ^2 = 10mmX10mm
35 = some volume resistivity value of ohms*Meters
0.006 = length in Meters

21,000 ohms - sounds like a realistic value

Did I do the math correctly?

Proud Mary

Sun Jan 13 2013, 05:57PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Tara, notice that the resistance figures in the data sheet are spread across a range, perhaps because of an uneven distribution of bubbles in the foam.

Because of the foam's heterogenous nature, perhaps it might be better to determine R empirically, by experiment, as there might be a significant difference of R between any one 100 mmÂ² sample and another.

There might also be an edge effect, if you imagine one edge bisecting a line of non-conducting bubbles, with such an effect become more significant as the size of the contact area is reduced.

Stella

tarakan2

Sun Jan 13 2013, 06:36PM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
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I will determine resistance empirically when I pay for it and buy it.

At this point I need to know the range of resistance values my hypothetical device will create so I can choose the best approach to the design or eliminate this idea all together.

Ash Small

Sun Jan 13 2013, 06:57PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I have some here, and some more in the shed. I've just been messing around with it and a DMM set to Ohms.

I thought maybe I could give you some figures, but a few problems come to mind. Firstly, on the Vermason website, it gives a range of values, and says the resistance is 'in between'. Secondly, I'm sure the stuff in the shed is damp, and this will obviously affect the resistance. I can't be certain that the stuff I have indoors isn't slightly damp either, due to humidity.

This raises the question 'How can you be sure that humidity won't affect the resistance of your proposed device?'

It will have to be in some sort of sealed, air-tight container, I suppose.

I also can't be certain that the stuff I have is the same specification as the stuff you propose to use, as it is just packaging that other stuff came in, although I suspect that it came from Vermason.

It's quite fun though, to see the resistance go down and up again as I squash it.

(Looks like this is my 2,000th post here

)

Dr. Slack

Mon Jan 14 2013, 10:14PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

If you design your force sensors correctly, you can make them read ratio rather than absolute, so detecting the ratio between a stressed and an unstressed portion of foam. This will make your circuit largely independent of the absolute resistivity. That way, you will be able to use foam from any manufacturer, or a dead bit kicking about in your component box, without worrying too much about its provenance.

OT, it is through carbon loaded foam that I discovered an arc is a short-circuit. I skewered a piece the size of a pencil eraser with the two cores of some bell-wire, and got about 100 ohms when I measured the resistance (YMwillV). I applied mains, and blew a 13A fuse.

tarakan2

Thu Jan 17 2013, 01:23PM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
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Posts: 179

A foam voltage divider...
I think that I got an idea of how that can be accomplished now.
Thank you.
It is a great suggestion.

Speaking of moisture, I may submerge my device in silicone. It is likely going to be sealed. But if I make it as a voltage divider, would it matter.

----------------------------------------- ---------------------------------

Actually a whole foam bridge. Not just a divider.

tarakan2

Sat Jan 19 2013, 04:08AM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179

In the circuit that I posted,
R1 and R2 in are in the scale of hundreds of Ohms
and
R3 and R4 are in the scale of hundreds of KiloOhms.

I am using this bridge to measure something, making R3 variable, but preserving the KOhm scale.

Would the relationship of R3 to the output voltage be linear, considering that R1, R2 have a lot less resistance than R3, R4?

I dropped out of college for health reasons so I effectively forgot all the mathematics so I cannot answer this question anymore.

Thank you.

tarakan2

Sat Jan 19 2013, 04:10AM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179

Bridge schematic.

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