LC oscillator to determine inductance

I have a wound coil about 3 feet long and an iron rod that goes through it (in and out). I want to measure the travel. Will a LC
oscillator (Colpitts?) work to determine how much of the rod is in the core by measuring the frequency? Is it linear?
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On 5/5/07 10:09 PM, in article snipped-for-privacy@4ax.com,

What frequency are you talking about? Is the rod solid? Laminated? Powdered?
Just about anything you measure will will be difficult to correlate with frequency other than by making an experimental table. Moreover, the inductance will be nonlinear.
Bill
-- Fermez le Bush--about two years to go.
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snipped-for-privacy@aol.com wrote:

Interesting experiment. This may help: http://myweb.cableone.net/adamsmed2/pto.htm
Ed
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wrote:

Thanks Ed, This is just an extension of my linear transformer idea., I ran into "production" problems and I decided to reduce the windings by 50%. (the transformer worked but it didn't last) It turns out frequecy measuring is easier than voltage measuring anyway.
Current plan is a 3/8 or 1/2" hot dipped galvanized rod transiiting a 3' section of 3/4" PVC with a buttload of 30ga windings on it (some good weather tolerant insulated wire this time) Can I deal with the lack of linearity by loading up the windings toward one end? I guess I really slept through reactive circuits in AC basics ;-)
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snipped-for-privacy@aol.com wrote:

You can experiment with that, but I'm not sure what would be most practical. That's a big coil - seems like it would be awkward to wind/rewind/rewind/rewind until you get it as good as you can. It might be easier to use a lookup table with a linearly wound coil.
How did you deal with the non-linearity with the coil that worked?
Ed

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wrote:

I was still playing with the PC interface when the coil failed. A little salt air destroyed magnet wire in a heartbeat. This time I am going with waterproof insulation and potting the whole thing. Absolute accuracy is not as important as repeatability. I was just curious what linearity problems I will have with a core that moves in an inductor. Is this log scale? I may end up making more than one. Fortunately I have a huge spool of the wire.
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snipped-for-privacy@aol.com wrote:

I don't know, but I would think is it is a whole bunch more linear than logarithmic. On small coils it is. If you don't need absolute accuracy, my bet is a regular winding will be fine.
Ed
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snipped-for-privacy@aol.com wrote:

Um, maybe! "Linear" transformers are the standard way to do this!

A coil can be made to determine the frequency an oscillator of some type.
But there are a number of problems here. The first question is do you HAVE to use an iron rod because of the rest of the application. A solid iron rod not only has frequency limitations (which means you'll probably have to use beaucoup turns of fine wire to get frequency down, but also has conductivity.
The conductivity of the rod causes the "metal detector" effect. With a standard oscillator type metal detector, the presence of conductors allows eddy currents which send the frequency higher by reducing the inductance of the coil (see brass screw example in the link someone posted) The zinc coating can help do this as well. The ferromagnetic properties of the rod will increase the inductance as it goes into the coil lowering the oscillator frequency. The two effects tend to fight each other. Best would be to use a ferrite rod in the coil which solves the problem. But doesn't solve the problem of where to get said rod! Custom ferrites are not so easy to find!
The final result won't be particularly linear but this is the 21st century and that's what microprocessors are for. And you didn't raise the question of accuracy. This kind of sensor will have a lot of accuracy drift as well.
Good luck!
Benj
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Thanks Ben. Brass indeed may be the way to go.
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wrote:

BTW the "brass" is not lost on me but I can live with steel if it gives me a better delta. This is going to be over salt water. Brass is certyainly attractive if expensive.
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