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Driving Helmholtz Coils - Best Practice

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Robert Miller

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Feb 12, 2017, 10:10:08 PM2/12/17
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The following questions are with regard to driving two coils arranged in
a Helmholtz configuration.

By convention, is a bipolar or unipolar signal applied? It would appear
the former would have the effect of doubling the emitted frequency due
to AC field reversal.

Are the signals most commonly applied in phase of out of phase? In terms
of field topology, what are the desirable effects of each method?

What is the difference in effect between having the coils magnetically
aligned (N facing S), and having them oppose (N facing N)?

Thanks for any insights along these lines.

Robert Miller

Spehro Pefhany

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Feb 12, 2017, 10:35:38 PM2/12/17
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On Mon, 13 Feb 2017 14:10:02 +1100, the renowned Robert Miller
<robert...@teledyne.com> wrote:

>The following questions are with regard to driving two coils arranged in
>a Helmholtz configuration.
>
>By convention, is a bipolar or unipolar signal applied? It would appear
>the former would have the effect of doubling the emitted frequency due
>to AC field reversal.

Whichever you like, depending on whether you want to change the field
direction or not. No it doesn't double the frequency- the field
changes direction. If you only measure amplitude you will get double
the frequency in the same way that looking at CFL flicker will get you
double the mains frequency. The magnetic field is a 3D vector field.

>Are the signals most commonly applied in phase of out of phase? In terms
>of field topology, what are the desirable effects of each method?

Out of phase with *what*?

>What is the difference in effect between having the coils magnetically
>aligned (N facing S), and having them oppose (N facing N)?
>
>Thanks for any insights along these lines.
>
>Robert Miller

They are oriented so that the fields are in the same direction (use
the right-hand rule). Direcion of winding and current direction
determine the field direction.

The purpose is to get a fairly uniform field in a volume between the
two coils.

BTW, ISTR that Helmholtz didn't quite get the optimum for many
applications you might find that a slightly different proportion is
better.

--sp


--
Best regards,
Spehro Pefhany

John Larkin

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Feb 12, 2017, 11:00:15 PM2/12/17
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This is a ODM, an oil debris monitor.

https://www.google.com/imgres?imgurl=https%3A%2F%2Fimage.slidesharecdn.com%2Fricknicolaasvbrturbinepartnerspaper-131029201114-phpapp01%2F95%2Fpaper-rick-nicolaas-vbr-turbine-partners-online-oil-debris-monitoring-as-an-effective-and-reliable-early-warning-system-for-bearing-damage-in-aeroderivative-gas-turbines-6-638.jpg%3Fcb%3D1383688666&imgrefurl=http%3A%2F%2Fwww.slideshare.net%2Finformaoz%2Frick-nicolaas-vbr-turbine-partners-paper&docid=ypWcisuHSUbV3M&tbnid=pce0v_ulHxr0xM%3A&vet=1&w=638&h=903&bih=630&biw=1313&ved=0ahUKEwif_uC2lYzSAhULqFQKHZntCgEQMwggKAQwBA&iact=c&ictx=1#h=903&imgrc=pce0v_ulHxr0xM:&vet=1&w=638

http://www.mdpi.com/sensors/sensors-14-06207/article_deploy/html/images/sensors-14-06207f1-1024.png


It's sort of an anti-Helmholtz coil. The outer coils are driven
anti-phase, so there is zero field in the center, at the pickup coil.
A little chunk of metal in the oil flow disturbs the field symmetry as
it zips through.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

Spehro Pefhany

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Feb 13, 2017, 12:50:53 AM2/13/17
to
On Sun, 12 Feb 2017 20:00:06 -0800, the renowned John Larkin
<jjla...@highlandtechnology.com> wrote:

>
>
>This is a ODM, an oil debris monitor.
>
>https://www.google.com/imgres?imgurl=https%3A%2F%2Fimage.slidesharecdn.com%2Fricknicolaasvbrturbinepartnerspaper-131029201114-phpapp01%2F95%2Fpaper-rick-nicolaas-vbr-turbine-partners-online-oil-debris-monitoring-as-an-effective-and-reliable-early-warning-system-for-bearing-damage-in-aeroderivative-gas-turbines-6-638.jpg%3Fcb%3D1383688666&imgrefurl=http%3A%2F%2Fwww.slideshare.net%2Finformaoz%2Frick-nicolaas-vbr-turbine-partners-paper&docid=ypWcisuHSUbV3M&tbnid=pce0v_ulHxr0xM%3A&vet=1&w=638&h=903&bih=630&biw=1313&ved=0ahUKEwif_uC2lYzSAhULqFQKHZntCgEQMwggKAQwBA&iact=c&ictx=1#h=903&imgrc=pce0v_ulHxr0xM:&vet=1&w=638
>
>http://www.mdpi.com/sensors/sensors-14-06207/article_deploy/html/images/sensors-14-06207f1-1024.png
>
>
>It's sort of an anti-Helmholtz coil. The outer coils are driven
>anti-phase, so there is zero field in the center, at the pickup coil.
>A little chunk of metal in the oil flow disturbs the field symmetry as
>it zips through.

Sort of an anti-LVDT too.

Robert Miller

unread,
Feb 13, 2017, 7:37:49 AM2/13/17
to
On 13/02/17 15:00, John Larkin wrote:

> This is a ODM, an oil debris monitor.
>
> https://www.google.com/imgres?imgurl=https%3A%2F%2Fimage.slidesharecdn.com%2Fricknicolaasvbrturbinepartnerspaper-131029201114-phpapp01%2F95%2Fpaper-rick-nicolaas-vbr-turbine-partners-online-oil-debris-monitoring-as-an-effective-and-reliable-early-warning-system-for-bearing-damage-in-aeroderivative-gas-turbines-6-638.jpg%3Fcb%3D1383688666&imgrefurl=http%3A%2F%2Fwww.slideshare.net%2Finformaoz%2Frick-nicolaas-vbr-turbine-partners-paper&docid=ypWcisuHSUbV3M&tbnid=pce0v_ulHxr0xM%3A&vet=1&w=638&h=903&bih=630&biw=1313&ved=0ahUKEwif_uC2lYzSAhULqFQKHZntCgEQMwggKAQwBA&iact=c&ictx=1#h=903&imgrc=pce0v_ulHxr0xM:&vet=1&w=638
>
> http://www.mdpi.com/sensors/sensors-14-06207/article_deploy/html/images/sensors-14-06207f1-1024.png
>
>
> It's sort of an anti-Helmholtz coil. The outer coils are driven
> anti-phase, so there is zero field in the center, at the pickup coil.
> A little chunk of metal in the oil flow disturbs the field symmetry as
> it zips through.
>

This is a virtual potential field and only applies if the anti-phase
signals are bipolar.

Single polarity anti-phase signals will produce a motional field between
the coils.

Robert Miller

Robert Miller

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Feb 13, 2017, 7:41:38 AM2/13/17
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On 13/02/17 14:35, Spehro Pefhany wrote:

>> Are the signals most commonly applied in phase of out of phase? In terms
>> of field topology, what are the desirable effects of each method?
>
> Out of phase with *what*?
>

The signal to each coil anti-phase toi the other.

Robert Miller

Robert Miller

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Feb 13, 2017, 7:44:07 AM2/13/17
to
On 13/02/17 14:35, Spehro Pefhany wrote:

>> Are the signals most commonly applied in phase of out of phase? In terms
>> of field topology, what are the desirable effects of each method?
>
> Out of phase with *what*?
>

The signal to each coil anti-phase with the other.

If bipolar, cancellation. If unipolar, a motional field.

Robert Miller

George Herold

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Feb 13, 2017, 8:54:51 AM2/13/17
to
With current in "the same direction" you get a fairly uniform
B-field between the coils. With currents driven in "opposite
directions" you get a fairly uniform field gradient (dB/dz) between the coils.

A field gradient puts a force on a magnetic dipole.
a uniform field puts a torque on a dipole.

George H.

John Larkin

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Feb 13, 2017, 11:38:16 AM2/13/17
to
Actually, 3-coil LVDTs are phased the same way. The central sense coil
has zero coupling to the seriesed outer coils when the movable core is
centered (or missing.)

It's common to drive the center coil and sense the voltage of the
series-opposed outer secondaries, but that's a detail. It's a
transformer.

How about such a structure as an audiophool phono pickup?
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