vibrating coil gradiometer

[tm]

George Herold a écrit :

Say you have an X,Y,Z orthonormal axis system
Your pick up coil will have a Z axis.
Then draw your "chopping" coil in the X-Y plane and have it rotating on,
say X axis. That way you'll have generate a rotating field in the Y-Z
plane (well for the loop on axis component).
Now you just keep the Z axis component of that for the pick up coil...

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.

 

[josephkk]

George Herold a écrit :

One way to do rotating coil would be with two coils:
One fixed and external one to sense the varying field and the other one
being a rotating short circuited ring.
The rotating ring will see an induced emf=w.B.s.sin(theta) which shorted
will induce a current in the loop, which in turn will produce a rotating
B component, which will be sensed by the enclosing sense coil.

I did not run the figures, you know how to do that, so I don't know if
the figures are practical or not.

A gradiometer could be build from this with two rotating rings and one
big enclosing coil, with turns in one sense for half its length, then
the same turns count in the other sense for the second half length.
You sense the field gradient emf across the whole coil...

Lock-ins should give you more than enough sensitivity, but again I did
not run the figures.

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

?-)

 

[Jasen Betts]

OK I'm still trying to get my head around how this works. But I think
I'm starting to see it. Even if this isn't used to measure field
gradients it might be fun to build one and see it work. "I'm not as
smart as other's and it's nice to have some data to help guide my
thinking"*

Yeah, sorry about that. Sticking my foot in my mouth again.
(But I have no shame and will certainly do it again.)

Do I gain anything by having multiple turns? If not then how about a
copper disk?

I was first thinking I wanted the stationary pickup coil with it's
axis along the field direction, but now I'm thinking it needs to be at
right angles?


And thanks for the 'crazy' idea Fred.
Do you know if this has ever been made?

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

 

[Jasen Betts]

Hi whit3rd, I've done that for a rough zero. Plenty good most of the
time when I only care about the field to a few percent or so. But I'm
contemplating a gradient measurement. I'd like to measure the
gradient over a length about equal to the sample size ~10 cm. And I
need to measure the gradients at the ~ 1uT/m (10mG/m... 1mG/10cm)
level. So something better than 1 mG. Now flipping over is fine...
(as long as you think carefully about how it's flipped) But a further
issue is that the field in the old building that I work in jumps
around at the few mG level... kinda randomly as elevators and fork
lifts move around. (I guess I could take the sensors to my house in
the country for zero adjustment.... then one worries about temperature
changes...) Anyway the mu-metal tube reduces the effects of a
changing local field.

ah, yeah gradients. for field strigth you gould use just an oblique
disk, but for gradient you'd need a saddle-shaped rotor

perhaps shaped z=x.y with the axle on the z axis (through the origin)
r=1.

yeah, it'll want a non-magnetic drive - perhaps air power or (as you
can measure and compnsate for frequency) just a glass flywheel

Yeah been there, all the brass bolts are screened for magnetic
effects beofre being used in the 'non-magnetic' instuments. (Old
carbon comp resistors are non-magnetic BTW)

hmm yeah, anything conductive that's moving greates a magnetic field.

 

[Fred Bartoli]

josephkk a écrit :

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

?-)

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

 

[[email protected]]

Ahh where am I getting this uniform field?  I'd like to sell this to
users so they can find a (roughly) uniform field region in their
building.















Hmm, I wonder if we are talking about the same sensors.  HMC1001 or
(HMC10xx)?

http://www51.honeywell.com/aero/common/documents/myaerospacecatalog-d...

(Or if that's too long find them at digikey)

The high sensitivity ones (HMC1001) have a gain that varies from 2.4
to 4.0 mV/V/G
 and a sensitivity tempco of ~0.3%/C... (much less with a current
drive, which I don't quite get.)

So matching two of these across B and T to the 0.1% level looks like a
lot of work.

I still like the idea of wiggling one back and forth.  That seems to
get rid of all this 'common mode' crap, and just give me the
difference that I want.  But I'm not sure how to wiggle it 10 cm and
keep it flat.

(ohh there's a noise graph (for the less sensitive flavor) on page 4,
1/f corner at ~100Hz.)

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).

 

[George Herold]

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.

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!

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

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

Thanks,
George H.

 

[[email protected]]

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.

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.

 

[Jeroen Belleman]

hmm yeah, anything conductive that's moving greates a magnetic field.

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

Jeroen Belleman

 

[whit3rd]

On 2012-11-03 16:15, Jasen Betts wrote:


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

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.

 

[Jeroen]

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

 

[George Herold]

josephkk a écrit :








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

--
Thanks,
Fred.- Hide quoted text -

- Show quoted text -

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.)

 

[George Herold]

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.

--
Cheers,
James Arthur- Hide quoted text -

- Show quoted text -

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.

 

[George Herold]

ah, yeah gradients. for field strigth you gould use just an oblique
disk, but for gradient you'd need a saddle-shaped rotor

perhaps shaped z=x.y with the axle on the z axis (through the origin)
r=1.

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

George H.

 

[Tim Williams]

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?)

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

 

[Jasen Betts]

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

prezactly!

cross section of magneit field detector with
rotating baffle.


:xxxxx:
:xxxxx:
: /:
: / :
============================/ :
: / :
:/ :
:.....:
:.....:


=== axle

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.

 

[George Herold]

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?)

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 addthat
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

Thanks Tim, I got to thinking that I could go back to the vibrating
coil idea, but now add a piece of ferrite in the coil. I don't want
the ferrite moving with respect to the coil... but have the two locked
together. Does the ferrite give me a voltage gain proportioanl to mu?
(the permability of the ferrite) If I could get another factor of
1000 or so then my paltry ~1uV signals might get to respectable mV
levels? (Where can I get a big piece of ferrite?... what type?)

George H.

 

[whit3rd]

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!

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.

 

[Tim Williams]

Thanks Tim, I got to thinking that I could go back to the vibrating
coil idea, but now add a piece of ferrite in the coil. I don't want
the ferrite moving with respect to the coil... but have the two locked
together. Does the ferrite give me a voltage gain proportioanl to mu?
(the permability of the ferrite) If I could get another factor of
1000 or so then my paltry ~1uV signals might get to respectable mV
levels? (Where can I get a big piece of ferrite?... what type?)

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

 

[George Herold]

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.

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.