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Resistivity. - Conductive packaging foam for IC chips.

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Ash Small

Sat Jan 19 2013, 05:34AM

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

I imagine that if you compress R3 and R2 by the same amount, your bridge would be more sensitive, and would also compensate for effects of moisture, as long as R1=R2=R3=R4, (ie, all identical pieces of foam) to start with, however, I'm not certain of this.

I'm assuming that you will measure voltage accross the bridge, and feed this voltage into a computer. I'm making assumptions here, and using no maths.

However, I can't be certain that it will automatically compensate for changes in humidity. This will have to be ascertained by trial and error (experiment).

You effectively have two voltage dividers, if that makes it easier to follow.

The 'classical' wheatstone bridge circuit, with different values, but the same ratio (R1/R2=R3/R4) only seems to make sense to me when Rx (R4 in this case) is not of the same order of magnitude (or not in the same range, at least) as the variable resistor used for R2. (eg, when using strain guages, etc., although this does work on the same principle as 'conventional' strain guages, although 'conventional' strain guages tend to increase resistance with increased strain.)

Maybe someone else can clarify?

(I dropped out of college due to lack of funds and work commitments, more than once, so can no longer do the maths either

)

tarakan2

Sun Jan 20 2013, 06:40PM

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

Ash Small wrote ...

I imagine that if you compress R3 and R2 by the same amount, your bridge would be more sensitive, and would also compensate for effects of moisture, as long as R1=R2=R3=R4, (ie, all identical pieces of foam) to start with, however, I'm not certain of this.

I'm assuming that you will measure voltage accross the bridge, and feed this voltage into a computer. I'm making assumptions here, and using no maths.

However, I can't be certain that it will automatically compensate for changes in humidity. This will have to be ascertained by trial and error (experiment).

You effectively have two voltage dividers, if that makes it easier to follow.

The 'classical' wheatstone bridge circuit, with different values, but the same ratio (R1/R2=R3/R4) only seems to make sense to me when Rx (R4 in this case) is not of the same order of magnitude (or not in the same range, at least) as the variable resistor used for R2. (eg, when using strain guages, etc., although this does work on the same principle as 'conventional' strain guages, although 'conventional' strain guages tend to increase resistance with increased strain.)

Maybe someone else can clarify?

(I dropped out of college due to lack of funds and work commitments, more than once, so can no longer do the maths either

)

Sounds like the most inquisitive population of the world are college dropouts. I am thinking of organizing a college dropout union (starting in the United States since that's where I live)

I was asking about the Wheatstone Bridge that was unrelated to foam. I won't need all four elements of the resistor to be made out of foam. In addition to that, using a Wheatstone Bridge implies that both '+' and '-' of the measuring device are not directly related to power and ground on the main circuit - something that may be impossible to achieve with a microcontroller circuit.

Ash Small

Sun Jan 20 2013, 07:25PM

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

tarakan2 wrote ...

I was asking about the Wheatstone Bridge that was unrelated to foam. I won't need all four elements of the resistor to be made out of foam. In addition to that, using a Wheatstone Bridge implies that both '+' and '-' of the measuring device are not directly related to power and ground on the main circuit - something that may be impossible to achieve with a microcontroller circuit.

OK. I 'assumed' you might feed the bridge with, say, 5V, so that, in my assumed circuit, the voltage accross the bridge would always be less than 5V. <5V seems a reasonable voltage to feed directly into a PC, or other microcontroller device.

Obviously just using one piece of foam would take up less space, but I was thinking using four identical pieces, and just compressing two (presumably sandwiched together, with an insulating layer in between might automatically compensate for changes in humidity.

How do you intend to deal with the humidity issue with just one piece of foam?

tarakan2

Sun Jan 20 2013, 08:47PM

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

Things are not so simple in my design.
Since I am building a wired glove, I was going to use upward and downward motion of the operator's finger.

So there would be two pieces of foam that will never be 'engaged' at the same time, for each finger of the wired glove input device. That makes an ideal voltage divider.

But I am not in a hurry to build anything. I spent too much money on a different version of this project already.
So now I am just learning about what is out there. I am planning to use a layer of conductive fabric (or a conductive fabric glove) and some strips of conductive foam. So my circuit will be defined by the design I am going to stay with.

There may be more to this design that I am not aware about at this point.
Just like I discovered the 'recipe' for cheap pressure sensors with conductive foam just by browsing the internet.

I had a number of alternative approaches in my mind.

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