vibrating coil gradiometer

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It's easier to keep the current in the coil nil (so it doesn't disturb the field), and instead just look at the induced emf (voltage signal, the 'electromotive force'). The voltage is proportional to time derivative of the dot product of B and A (where A is the directed area of the coil, an area multiplied by the unit coil-axis vector).

So, any fluctuation of the area during the movement is just as much a signal as the B-induced AC emf signal. Uniform rotation won't flex the coil much, but simple vibration might.

The axis of rotation's B field component cannot be sensed by this kind of magnetometer; you get only the other two B components, from phase and amplitude of the emf.

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.

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.

Tim

Thanks whit3rd, Fred said there'd be a signal at 2f, and I was trying to understand how there could be another minimum when the rotating coil axis was aligned with the field. (I sorrta figured it out while I was writing, so maybe I shouldn't have posted it.)

George H.

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

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.

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.

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good enough)

The cheap answer is like a Pringles.

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.

Cheers

Phil Hobbs

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'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.

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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

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

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.

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

enough)

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.

try "z=xy"

I'm talking about orienting the toroid edgeways, so that the solenoidal turn of the toroid is also orthogonal to the pickup solenoid. You probably lose a factor of 2 or 3 in sensitivity that way, but you win by reducing the crosstalk.

Any small crosstalk can be tweaked out fairly easily.

Cheers

Phil Hobbs