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I am planning to build a grip-release sensor using the foam. I'm pretty interested in the way it works. Does the resistance in circuit increase or decrease when force is applied onto it? And this will be proportional to the force in what manner? I will need to apply a potential difference won't I?

pipe
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Karl Stark
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    I would not take anyone's word for it until I would have tried it out myself. It is possible that their foam is different from mine. But **dissipative** foam (the pink stuff) might not conduct at all. Usually it is only treated such that a charge cannot build up. For conductivity I think you would need the black **conductive** foam. – Bimpelrekkie Aug 24 '17 at 12:11
  • Sorry yes I think I meant that. Any idea on how that would work? I don't have access to my electrical resources now – Karl Stark Aug 24 '17 at 14:22
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    Instead of using the pink foam, use black (conductive) foam. Arsenal proved with his experiment that the resistance can change under force. Conductive foam contains conductive materials like carbon. When pressure is applied then those carbon particles might be press more closer to each other and that might provide more paths for electricity to flow and lower the resistance. – Bimpelrekkie Aug 24 '17 at 14:31
  • So the more force applied the lesser the resistance for these conductive foams? – Karl Stark Aug 24 '17 at 14:44
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    My measurements indicate it behaving just the other way round, just like any conductor would when you make the cross section smaller, but I expected it also to get smaller when pressure is applied. – Arsenal Aug 24 '17 at 14:46
  • That's what I would expect however, in Arsenal's experiment the resistance went up a bit so perhaps my hypothesis is wrong. You just need to try it out in your setup and see what happens and if the resistance change (if any) is enough to detect reliably. – Bimpelrekkie Aug 24 '17 at 14:46
  • Hmm... I understand. The thing is I may not be able to conduct tests for a while and o just wanted to understand the physics behind it. Arsenal used a dissipate foam, would that make a difference comparing to the conductive foam? – Karl Stark Aug 24 '17 at 15:00
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    I used both and the dissipative was not usable. – Arsenal Aug 24 '17 at 15:49

2 Answers2

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Interesting idea!

Well I just tried it. I hooked up my trusty Keysight 34410A to the test leads and pierced what I think is dissipative foam (pink foam of an electronics shipment). The ohm reading was overload, so no measurable resistance. Which is to be expected like Bimpelrekkie suspected.

Dissipative material is just too high resistance to make a usable measurement with. I guess with some high voltage equipment you'd get a value, but a grip release sensor sounds like someone is touching it, so high voltage is probably not the way to go.

But I also had some conductive foam (black stuff, quite stiff) lying around. It's a sheet of 30 x 10 x 0.8 cm. When I pierced it at the end, so the whole 30 cm where between the probes, I measured around 20 kOhm at first but that was dropping off the longer I had the probes in.

It didn't really settle over a time of several minutes, so I'll leave it in and see where it goes.

To see if it is pressure sensitive I pushed with the isolated back of a screwdriver onto the foam. The value went up by around 80 Ohm, from 17610 Ohm to 17690 Ohm, after releasing the pressure the value went down 30 Ohm immediately after release and then dropped back in a few seconds.

The screwdriver was rather small, around 1 x 1 cm, so a bigger one would give a higher increase.

Right now it doesn't seem to be a rock stable system but I can imagine you can get something out of it with some clever algorithm. Especially since you are interested in a release, the absolute value might not matter but a change over a short period of time.

After more then an hour it has settled at around 16889 Ohm. As I was squeezing it before I started the experiment, it might have been the time it needed to restore its original structure completely.

That seems quite plausible, after squeezing it again (gripping it in the middle) the resistance went back up to 20 kOhm and is starting to go down again.

Here is a data log of a squeeze:

Data log of a squeeze of conductive foam

As you can see, it really has a long recovery time to get where it originally was. I can't say how many cycles of squeezing it will survive. So you have some tests ahead of you.

Arsenal
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Here is my theory. The carbon-impregnated foam can be considered as a bunch of interconnected small resistors, a complex randomly connected resistor network. Foam cells form a characteristic size of network sections.

In first approximation the impedance of this network shouldn't depend on the network deformation, since individual small resistors (walls of foam bubbles) do not change.

However, when a stronger compression force is applied, some resistors might create shorts, but some subsections might break up. So the net effect is impossible to predict. If more sections break relative the number of collapsed cells, the impedance will increase. If more foam cells collapses, overall impedance will go down. If some broken sections recover their initial shape and restore electrical contacts, the impedance will re-bound to some degree. The whole process will likely deteriorate if more cycles of pressure is applied.

More, foams may have different cell structure. There are "high-density" foams with closed set of cells, and there are foams with loose cell structure. Behavior of total impedance will likely differ a bit.

In sum, the conductive foam is not the best sensor of applied pressure.

Ale..chenski
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