On 5/5/07 10:09 PM, in article email@example.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.
-- Fermez le Bush--about two years to go.
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
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 ;-)
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
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
Um, maybe! "Linear" transformers are the standard way to do this!
A coil can be made to determine the frequency an oscillator of some
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.
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