vibrating coil gradiometer

[George Herold]

Permeability to the outside world is only slightly higher than 1, because
it's necessarily an open magnetic loop. A solid toroid will give you
about mu times the coil's air-cored inductance, but that doesn't help much
because none of that flux was gained from the outside world.

Yeah no torroids.... unless you're making a flux gate thing.

Since effective permeability is so low, material doesn't matter -- #33
(reasonably flat mu tempco, unremarkable otherwise) and #61 (lower mu,
high frequency, average tempco) rods are readily available from Amidon,
and I think other types.

So you're saying there will be very little voltage gain, with a
ferrite plus coil, vs just the coil? (I must admit I find magnetic
materials a confusing lot.) That's OK I hopefully won't make the same
mistake twice.

George H.

 

[Tim Williams]

So you're saying there will be very little voltage gain, with a
ferrite plus coil, vs just the coil? (I must admit I find magnetic
materials a confusing lot.) That's OK I hopefully won't make the same
mistake twice.

Oh, there's gain, just not mu times -- should be roughly the ratio of
lengths, core to coil. So a 6" loopstick in an AM radio works a lot
better than the teeny winding on it, etc.

Tim

 

[George Herold]

Oh, there's gain, just not mu times -- should be roughly the ratio of
lengths, core to coil.  So a 6" loopstick in an AM radio works a lot
better than the teeny winding on it, etc.

Tim

Hmm OK thanks, If I want to shake it around it's not worth the extra
mass... which I guess leads back to James's original idea.

George H.

 

[josephkk]

Hi Jasen,

Let's start with the simple baffle for field detection. Is it a
disk? Or long in one dimension and narrow in the other.... like a
popsicle stick.

I don't suppose you have a reference for the saddle shaped 'gradient'
baffle.
(a good picture would help me a lot.)

The cheap answer is like a Pringles.

http://en.wikipedia.org/wiki/Paraboloid

George H.

 

[George Herold]

One approach would be to put a toroid edgewise inside a solenoidally
wound coil, and saturate it with the toroidal field.  It should be
possible to do that with very low crosstalk, if the axes of the toroid
and solenoid are perpendicular.

'scratch scratch'... You're riffin' on James's idea? Isn't this like
the fluxgate that Jan made (and posted here) a while back? (I've half
forgotten how that works.)

George H.

 

[George Herold]

IIRC fluxgates drive one winding hard via another transformer, and look
at the duty cycle shift of the pulse in the sense winding, which tells
you what the external field is.  Or is that a different technique?

I was just thinking of a way to turn the core on and off a la James,
while keeping the crosstalk down at the 1E-6ish level.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -

- Show quoted text -

Hmm, maybe we are talking about different geometries.
Googling "toroid fluxgate" I get this,

www.geotech1.com/pages/mag/projects/fmx1/fmx1.pdf

The first figure is what I was picturing. I remember going over this
in my mind when Jan posted it, and I thought I got it.... but I'm
going to have to draw pictures again.
(The pickup coil sees a signal at twice the drive frequency, IIRC)
Say Jan, if you're lurking do you have a good reference for the
toroidal fluxgate?

George H.

 

[Tim Williams]

I was just thinking of a way to turn the core on and off a la James,
while keeping the crosstalk down at the 1E-6ish level.

Hmm, nonuniformity in the toroid is on the order of N turns; you'd need
over 1000 turns, in a single even layer, to get to that level I'd think.

Hmm, nah, it wouldn't quite be that bad, nor that easily defined. Each
turn acts like a dipole at a slightly different angle. After ten turns,
you have a dodecapole in a circle. A suitable distance from that and your
far field drops off ridiculously fast (that is, ignoring the solenoidal
component of a standard progressive winding), isn't it ~1/R^N for an
N-pole?

So it comes down to the compromise between relative size (how much space
you can get between the toroid and solenoid), near field nonuniformity
(number of turns, and their distribution), and the amount of with-core vs.
without-core gain you experience from using the toroid as a bar magnet.

I'm carrying out a thought experiment which seems to suggest there's no
free lunch here, i.e., it reduces to a linear/folded form of the "no free
lunch" once-you-saturate-it-the-control-winding-couples-in hypothesis.
It's just an hypothesis, but there may be a theorem in there.

Tim

 

[Jasen Betts]

Hi Jasen,

Let's start with the simple baffle for field detection. Is it a
disk? Or long in one dimension and narrow in the other.... like a
popsicle stick.

This is soething I've just thought up.

someone said a rotating coil coupled to the leads by a coaxial
transfromer (like found under VHS drums)

someone else made a remark (that I may have misread) that started me
thinking that you could stir the magnetic field and make it pass over
turns of a stationary coil by combining the two rotating coils into
a single stirrer


given that the inner radius of the coils is 1 unit

For the field baffle with the axle on the Z axis though the origin
the surface shhould approximate the plane z=y (where xx+yy < 1)

on further thought most of the current will flow in the edges of the
baffle, so, a simple inclined elliptical ring would probably work as well
as he disc does.

I don't suppose you have a reference for the saddle shaped 'gradient'
baffle.
(a good picture would help me a lot.)

try "z=xy"