Variable capaciance

[Smittysmith]

Any ideas?? I have a load cell and i want to build an appropriate circuit that would read the change in deflection proportional to the force applied to the load cell. the capacitor plates are situated on in the load cell and when the beam deflects the CSA of the plates change and would give a change in capacitance. I need to attach the circuit to an lcd screen to read the weights.
A wheatstone bridge cicuit using capaciors, would that work??? Help would be greatly appriciated

 

[crutschow]

What is the capacitance value and the change in capacitance that you want to measure?

 

[Reloadron]

Can you share the Model Number and Part Number of the load cell you have as well as the manufacturer? That would sure help a lot. Loadstar is popular for capacitive load sensors. I generally have used resistive sensors.

Ron

 

[Smittysmith]

more info

4.74 micro farads, i suppose i would need a balance circuit of some sort. it's a home made load cell. Seeing how effective capacitors are as the trnsducer. I have to weigh about 50N f beam max deflect .5 of a milimeter

 

[crutschow]

So you don't know how much the capacitance will change with load?

 

[Smittysmith]

Well depending on the load, it will change the capacitance. do you have a preliminar one that I could add values to. would it be same structure even if it was 5kg or 500kg. form about 4.7 micro farads to 5 or 6

 

[Roff]

4.74 micro farads, i suppose i would need a balance circuit of some sort. it's a home made load cell. Seeing how effective capacitors are as the trnsducer. I have to weigh about 50N f beam max deflect .5 of a milimeter

I can't imagine any beam structure that would have that much capacitance. Maybe 4.74 picofarads.

 

[Smittysmith]

No dude I made a load cell from a rectangular block. Milled through the centre of it. Leaving 3mm of material to deflect. I fixed small plates to act as capacitors to it. when the beam deflects the plate attached will too. there is a plate fixed to the top and bottom. air is what is between them too

 

[Roff]

No dude I made a load cell from a rectangular block. Milled through the centre of it. Leaving 3mm of material to deflect. I fixed small plates to act as capacitors to it. when the beam deflects the plate attached will too. there is a plate fixed to the top and bottom. air is what is between them too

How do you know it's 4.74uF?

 

[Smittysmith]

dude I'm not answering questions all day about this thing. all i need is a schematic of some type of circuit that may work. if anyone can help it would be greatly appriciated. the common CSA if the capacitors without a load is Plate area: 2.7mm2 Distance is .5mm and dielecric const. of air is 1. just a drawing that could work. i've tried connecting the capacitors to 2 occollators then connecting them to a D type flip flop F1-F2 diference is linear and gives a digital reading. Would a wheatsone bridge with capacitors work if i had an A/D converter??? get the balance right???!!!!!

 

[Roff]

Dude, you won't get much more help with that attitude. I'm not just some nosy old fart. The reason I asked was because, as I said, 4.74uF didn't pass the smell test.
The equation for a parallel plate capacitor is εA/d, which I think you know.

However, ε is the absolute permittivity of the dielectric, not the relative permittivity (which you mentioned). The value of ε is 8.854e-12 F/m.
You must have used some number other than 1 for ε, but I am baffled as to how you came up with C=4.74uF.
If I understand your dimensions correctly, your dimensions are A=2.7e-6m² and d=0.5e-3m. If this is correct, your capacitance is 47.8e-15 Farads, which is .0478 picofarads. This is not a measurable capacitance in your setup.

 

[crutschow]

Dude lose the 'tude.

 

[Reloadron]

also.

Considering what you are trying to measure I would have considered using a resistive strain gauge in a bridge configuration.

Those asking questions are asking them for good reason, because your numbers don'y make sense. Anyway best of luck using capacitance.

Ron

 

[Smittysmith]

i used an online calculaor for getting the acpacitance Parallel Plate Capacitor Capacitance Calculator by inputing the area as 2.7 and the distance as .5 and the dielectric const as 1 i got 4.78 uf. the wein bridge looks pretty ideal. and pico would be that with 6 decimal changes left.

sorry for the attitude old dude

 

[JimB]

Try the capacitor calculator again and look carefully at the answer, it is not 4.78uF but 4.78 e-8 uF.
Or, 0.0478pF as Roff calculated earlier.

JimB

 

[Warpspeed]

i used an online calculaor for getting the acpacitance Parallel Plate Capacitor Capacitance Calculator by inputing the area as 2.7 and the distance as .5 and the dielectric const as 1 i got 4.78 uf. the wein bridge looks pretty ideal. and pico would be that with 6 decimal changes left.

sorry for the attitude old dude

No you didn't!
Those numbers give the answer 4.78 e-8 uF, which is .0000000478uF
It also says 0.0478pF which is the same value.

Good luck measuring any changes to something that small.

 

[Smittysmith]

No you didn't!
Those numbers give the answer 4.78 e-8 uF, which is .0000000478uF
It also says 0.0478pF which is the same value.

Good luck measuring any changes to something that small.

Thank you very very much sir. That was the answer i was looking for. Thank you. Could I get a square wave from that?? get an old digital reading aye?

 

[Warpspeed]

Trying to do anything at all with something that small (electrically) is just not possible.

It needs to be at minimum a thousand or ten thousand times larger before you can even begin to measure it electrically by measuring capacitance, with any sort of accuracy or resolution.

Not trying to be rude, but suppose I asked you how can I measure sub micron sizes with a wooden school ruler, what would you advise me to do ?

 

[Smittysmith]

Trying to do anything at all with something that small (electrically) is just not possible.

It needs to be at minimum a thousand or ten thousand times larger before you can even begin to measure it electrically by measuring capacitance, with any sort of accuracy or resolution.

Not trying to be rude, but suppose I asked you how can I measure sub micron sizes with a wooden school ruler, what would you advise me to do ?

Sorry I replies to the wromg messege. Eh yeah I cannot find the reply now but did you not say that I should have it 40mm by 40mm and 3mm gap?? and the capacitance determines the frequency that can be connected to oscillators. then i asked could i get a square wave or am I going crazy