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Geesh you're no help at all. :^)

OK I'll have to try and mock something up. I've got a copper disk on a stick, but spinning it with a power drill was a no-go. (As you might have guessed the power drill spits out all sorts of B field stuff.)

We can't afford to patent anything.

George H.

BTW it occurred to me that the picked up voltage should be at 2 times the rotating frequency, so you'll have to lock on 2nd harmonic.

Was joking, of course :-)

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Good I somehow had the same picture.

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OK I'll have to try and mock something up. I've got a copper disk on a stick, but spinning it with a power drill was a no-go. (As you might have guessed the power drill spits out all sorts of B field stuff.)

Spin it with compressed air. Like the old A/C vacuum powered gyros.

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Thanks Fred. Just the idea got me to thinking along other lines for other applications.

?-)

sounds reminiscet of a Lesley rotating loudspeaker, except it's a magnetism transducer instead of air pressure.

He he... The idea came because for another application I won't divulge, which even don't have any rotating loops, my head is full of rotating B vectors, curls and such, turning allover in space... When you feel like between a juggler and an acrobat and your toys are vectors, it come easy :-)

The Leslie speaker doesn't rotate, the baffle does.

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An off-the-wall idea: suppose you vibrated the pickup coil electrically rather than mechanically?

Fluxgates work by periodically saturating a core, making it magnetically disappear from inside a pickup coil.

Perhaps you could use the same technique to make the pickup coil appear to physically translate? Use two concentrating / field sampling cores, one pickup coil. Alternately disable one core, then the other? Or, in the degenerate case, just two fluxgate magnetometers, spaced.

(I'm deep in other problems now, so I'm throwing this out less-than- half-baked).

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Hmm, that's cool too. But aren't I still trying to balance two coils (sensors). But maybe that's the way to go? I could imagine a 180 degree spinner and a balance knob.

I'm still thinking about a vibrating sensor.

Friday at the end of the day, I called the family and asked them meet me at a tavern for food and drink. When they arrived I was drawing pictures of a pendulm type thing. (The plane of rotation needs to stay prependicular to gravity, but I picture a driven oscillation at various angles.) I said I didn't know how to make it vibrate, or oscillate non- magnetically, my son asked for how long, and suggested a human powered motor!

At the minimum maybe a wind up spring!

All ideas, no matter how baked are welcome...

Thanks, George H.

It could be done with a single pickup coil, IFF you can make the magnetics work. That's the part that needs puzzling out.

The basic idea is that if you add, then remove section A. below, you effectively translate pickup coil B. within the ambient field.

C. core | | / _____ ___|||||___/ | | ||||| | |_____|___|||||___| \ | | \ \ A. B. switched pickup core coil

You can do that electrically with the fluxgate technique of saturating A. in an orthogonal direction (so as not to affect the pickup coil's signal).

Again, no practicalities have been addressed; this is raw.

That doesn't fit into my frame. Please explain.

Jeroen Belleman

A moving conductor in a magnetic field gets induced eddy currents. Those currents, in turn, create a small magnetic field (opposed to the ambient field). Another way of thinking about it, is that conductive materials are all diamagnetic (exclude magnetic field lines from their volume), to varying (AC) magnetic fields. The trouble is, direction-of-field variation is the same, as far as the induced currents go, as an AC applied field.

So, if you make a flip-coil magnetometer, it's important that there isn't any large volume of metal (except for the coil) in motion near the sensing tip.

OK, I grant you that I perhaps removed a bit too much of the context. My point was that a conductor moving in a field-free region doesn't generate a magnetic field all by itself. Of course, it *will* distort fields it moves through. In that sense, it could be said to generate a field.

Jeroen Belleman

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Hi Fred, So your idea has been going 'round in my head. (PI) As I get it, there's a current induced in the rotating coil. The current's proportional to the changing flux, and the current is constrained to flow in the plane of the loop. So the induced magnetic moment (MM) has a changing angle, (and magnitude.)

Now it's easy to see that the current goes to zero, when the plane of the loop is parallel to the field. I have a bit of a harder time seeing what goes on, when the loop axis is aligned with the field. At this point the induced MM is maximal, Oh I think I see, the pick up coils emf goes as the change in the field!

(So maybe the pickup coil at 45 degree's Earths field?)

George H.

Hmm, if the rotating coil was off axis, there'd be some 'small' asymmetry in the signal (from a non-uniform field). (And a big asymmetry with the whole thing not centered in the pickup coil.)

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Hi James, That's cool! But I'm not sure how the ferrite works in the Earth's field. Does a ferrite 'take up' more magnetic field space, than just it's physical volume? (If I stick a piece of ferrite next to my hypothetical gradiometer will I get a signal?)

(I'm also not sure how to turn off the one section and not have it leak.)

Hey what if I just shake a piece of ferrite back and forth in a coil? (A vertical rod on a spring may be good enough if you're not near the magnetic 'equator'.)

I was just trolling digikey for documents, and I can get a two axis sensor from honeywell for ~$7 in ones. ($18 for three axis)

Dang, I'll have to shake at least one thing soon,

George H.

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Interesting, (a bit cryptic) Are you saying I can shape a rotor in a static coil to give a field gradient measurement?

George H.

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You know a loopstick antenna? The length of the permeable (typically ferrite) rod intercepts some fraction of the B-field of a propagating radio wave. It's the magnetic dual of a mag-loop antenna, which is itself the E-M dual of a dipole. Consider:

- A [short] dipole intercepts some fraction of the electric field of a propagating wave. Any E-field pointing along the axis of the antenna polarizes it, inducing a voltage.

- A [small] loop intercepts some fraction of the flux of a propagating wave. Any B-field pointing through the center of the antenna induces a current.

- We can enhance the amount of flux entering the loop by adding a permeable material, which "sucks in" nearby fields (not strictly true, but close enough for hand-waving).

- Ironically, the result is a dipole again, but a magnetic dipole: it's sensitive to B-field along the axis of the core (assuming typical geometry, like a cylindrical "loop stick").

Now, that's all well and good but it doesn't work for static fields, because the stick isn't moving (intercepting different regions of B field).

Suppose you stick a strong magnet onto a the protruding end of the ferrite rod, enough to saturate it. The saturated part no longer functions as ferrite, so the rod appears shorter. The influence from external fields has changed. Now change the magnet to an electromagnet and turn it on and off. The influence from external fields alternates with the saturable section.

The only rub -- and many people forget this about saturable reactors -- is, once you saturate one part of the core in the assembly, the whole thing now ceases to be balanced, and flux from the control winding couples into the sense winding. The same will occur here, where a lot of MMF (magnetomotive force, amp-turns) is dropped across the reluctance of the saturated piece of ferrite (which now has very high reluctance). The control section ceases to be contained in a core, so flux leaks through the air and screws everything up. And obviously, the error is in phase with the measurement, so you can't separate the measurement from the error.

I'm not damning the approach -- you could, for example, attempt to shield the control winding with extra pole pieces, or a good conductor. Maybe the induction can be balanced with a winding not around the core (ooh, that could be tricky since we're trying to measure small differences in the first place!). Not sure how well that would work. I'll also add that I forget how fluxgates work. It could be they already do this (as mentioned earlier), which would make the solution a whole lot easier to imagine.

Tim

prezactly! cross section of magneit field detector with rotating baffle. :xxxxx: :xxxxx: : /: : / : ============================/ : : / : :/ : :.....: :.....:

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xxx coil windings ... coil windings //// baffle (ideally superconducting, but copper or aluminium is probably good enough) the sloped planar baffle detects fields. a saddle shaped baffle will detect gradients.