Measuring magnetic field

[eromlignod]

Hi guys:

I'm working on an application where I need a simple DC coil that
provides a particular magnetic field strength. I have to experiment to
determine just how strong the field needs to be. So I have wound an
arbitrary coil to experiment with. I will vary the voltage on it until
I experimentally achieve the necessary field in my application.

Now when I know that this coil is producing the right field, how can I
measure it? If I can get a measurment of the field intensity, I can
wheel-and-deal mathematically with different voltages, wire gauges and
dimensions of a coil so that I can design one to meet the field
requirement.

Is there a simple way to do this without expensive equipment? I
suppose I could take apart my arbitrary test coil and calculate what
the field must have been, working backwards, but is there a more
direct, emperical way? I want to be able to say, "My application
requires ______ teslas".

Don
Mechanical Engineer

 

[John Popelish]

eromlignod wrote:

Hi guys:

I'm working on an application where I need a simple DC coil that
provides a particular magnetic field strength. I have to experiment to
determine just how strong the field needs to be. So I have wound an
arbitrary coil to experiment with. I will vary the voltage on it until
I experimentally achieve the necessary field in my application.

Now when I know that this coil is producing the right field, how can I
measure it? If I can get a measurment of the field intensity, I can
wheel-and-deal mathematically with different voltages, wire gauges and
dimensions of a coil so that I can design one to meet the field
requirement.

Is there a simple way to do this without expensive equipment? I
suppose I could take apart my arbitrary test coil and calculate what
the field must have been, working backwards, but is there a more
direct, emperical way? I want to be able to say, "My application
requires ______ teslas".

Don
Mechanical Engineer

A coil produces voltage in proportion to the rate of change of flux

passing through the hole. So, unless you have a way to spin the
sensor coil around to convert the DC field to AC, it is not going to
give you a signal. You might get a linear hall effect sensor like:
and calibrate it with a Helmholtz coil. There are simple formulas
that relate the field strength in the center of the coil versus the
number of turns and the amperes passing through those turns.
http://www.allegromicro.com/hall/linear.asp
http://www.netdenizen.com/emagnet/helmholtz/idealhelmholtz.htm

 

[eromlignod]

John Popelish wrote:

A coil produces voltage in proportion to the rate of change of flux
passing through the hole. So, unless you have a way to spin the
sensor coil around to convert the DC field to AC, it is not going to
give you a signal. You might get a linear hall effect sensor like:
and calibrate it with a Helmholtz coil. There are simple formulas
that relate the field strength in the center of the coil versus the
number of turns and the amperes passing through those turns.
http://www.allegromicro.com/hall/linear.asp
http://www.netdenizen.com/emagnet/helmholtz/idealhelmholtz.htm

I don't think I explained myself very well. I'm *applying* DC voltage
*to* the coil to produce a magnetic field, like an electromagnet. The
coil has a ferrous core. I just need to measure how strong this field
is (polarity does not matter) so that I can reproduce the same field
with a coil of different design.

I guess I could rent a gaussmeter, but it seems a shame just to take
one simple measurement.

Don

 

[GregS]

In article <1121883006.397199.236630@g49g2000cwa.googlegroups.com>, "eromlignod" <eromlignod@aol.com> wrote:

John Popelish wrote:

A coil produces voltage in proportion to the rate of change of flux
passing through the hole. So, unless you have a way to spin the
sensor coil around to convert the DC field to AC, it is not going to
give you a signal. You might get a linear hall effect sensor like:
and calibrate it with a Helmholtz coil. There are simple formulas
that relate the field strength in the center of the coil versus the
number of turns and the amperes passing through those turns.
http://www.allegromicro.com/hall/linear.asp
http://www.netdenizen.com/emagnet/helmholtz/idealhelmholtz.htm


I don't think I explained myself very well. I'm *applying* DC voltage
*to* the coil to produce a magnetic field, like an electromagnet. The
coil has a ferrous core. I just need to measure how strong this field
is (polarity does not matter) so that I can reproduce the same field
with a coil of different design.

I guess I could rent a gaussmeter, but it seems a shame just to take
one simple measurement.

Don

Might be tricky, but waving another sense coil at a specified
speed through the field at a specified distance and rotation,
will give you a relative measurment
for the other coil.

greg

 

[Don Taylor]

"eromlignod" <eromlignod@aol.com> writes:

I'm working on an application where I need a simple DC coil that
provides a particular magnetic field strength. I have to experiment to
determine just how strong the field needs to be. So I have wound an
arbitrary coil to experiment with. I will vary the voltage on it until
I experimentally achieve the necessary field in my application.

Now when I know that this coil is producing the right field, how can I
measure it? If I can get a measurment of the field intensity, I can
wheel-and-deal mathematically with different voltages, wire gauges and
dimensions of a coil so that I can design one to meet the field
requirement.

Is there a simple way to do this without expensive equipment? I
suppose I could take apart my arbitrary test coil and calculate what
the field must have been, working backwards, but is there a more
direct, emperical way? I want to be able to say, "My application
requires ______ teslas".

Don
Mechanical Engineer

If wildly high accuracy isn't needed, how about using the earth's
magnetic field, your coil's field and a suitably placed compass?

Orient your coil so that it is at right angles to magnetic north,
position the compass, and see how large the deflection is from
"true north." For example, if your field, at a given distance
was equal to the earth's field then the compass would be deflected
45 degrees. Using deflections near 0 degrees or 90 degrees would
be less accurate but if you could arrange to have a deflection
that was somewhere between those then this might give you an
estimate of your field, be easy to repeat, and inexpensive.

You could also buy hall effect sensors, small solid state devices
that are available giving an output linearly proportional to the
magnetic field they are in. There are some available that are
non linear or that just switch on and off, so watch the specs.

You might also search Ebay for gauss meter or gaussmeter, sometimes
there are relatively cheap meters available there.

 

[Mike]

You might want to take a look at at the Honeywell/Microswitch SS94
series of linear hall effect sensors. They have them to directly
measure static fields from +-500 up to +-5 Kilo-Gauss. They provide a
ratiometric output. A DVM and regulated power source is all you really
need to get a decent reading. The price is around $15 ea. They are a
little hard to find in unit quantities, but can be found.

Mike

 

[Robert Scott]

Don,

I thought your self-tuning piano was further along than this. Or is
this a new project?


-Robert Scott
Ypsilanti, Michigan

 

[Phil Hobbs]

Mike wrote:

You might want to take a look at at the Honeywell/Microswitch SS94
series of linear hall effect sensors. They have them to directly
measure static fields from +-500 up to +-5 Kilo-Gauss. They provide a
ratiometric output. A DVM and regulated power source is all you really
need to get a decent reading. The price is around $15 ea. They are a
little hard to find in unit quantities, but can be found.

Mike

Hall sensors will work for measuring the field in the air. Measuring
the field inside the iron is not an easy matter, and the interaction of
an iron-core electromagnet with a nearby ferromagnetic object is quite
complicated--you may not be able to transfer the results to a different
coil design. The basic problem is that the magnetization in the object
induces a magnetization in the iron core, which induces magnetization in
the object, and so on...and it's all nonlinear.

Cheers,

Phil Hobbs

 

[Andy]

Andy writes;

This is a very innovative idea and I think Don's approach is a good
way for
a home experimenter to check out his Helmholtz coil, if "approximate"
accuracy
is good enough.

There are websites which show the earth's magnetic field over the
globe,
with fair accuracy, and these will give you what number, in
microTeslas, to
use for your location.

If you can't find it, use 50 microTeslas, which is a pretty good
round
number for the earth's magnetic field in the US.

When playing with the compass, turn the coil's current up much
higher than needed to make sure the needle will go 90 degrees to
the "non energized coil" reading, so that you know you are at
right angles to the earth's field. The magnetic deviation changes
a BUNCH throughout the US, so an eyeball alignment may not be
particularly good. Then , just back off the coil current till you
get 45 degrees, and the coil field AT THE COMPASS will be equal
to the earth field. Then, you can use this as a calibration point......
When you compare it to the equations ( B = uH, etc) it should
work out very close to 50 microTeslas. If it is way off, then you
probably used meters instead of feet or furlongs instead of
fortnights,
or some other such dumb error that we all do when we do it for the
first time.........

Anyway, I ran an Electromagnetics Lab at Raytheon for a long time,
and learned to have confidence in those weird calculation of turns,
amperes,
diameters and areas..... all that stuff really works, tho it's REAL
EASY to
make a mistake in using the wrong units.......

Good luck, and kudos to Don for suggesting a simple way to avoid
buying
expensive equipment.....

Andy in Texas

 

[Rene Tschaggelar]

eromlignod wrote:

Hi guys:

I'm working on an application where I need a simple DC coil that
provides a particular magnetic field strength. I have to experiment to
determine just how strong the field needs to be. So I have wound an
arbitrary coil to experiment with. I will vary the voltage on it until
I experimentally achieve the necessary field in my application.

Now when I know that this coil is producing the right field, how can I
measure it? If I can get a measurment of the field intensity, I can
wheel-and-deal mathematically with different voltages, wire gauges and
dimensions of a coil so that I can design one to meet the field
requirement.

Is there a simple way to do this without expensive equipment? I
suppose I could take apart my arbitrary test coil and calculate what
the field must have been, working backwards, but is there a more
direct, emperical way? I want to be able to say, "My application
requires ______ teslas".

I'd calculate it.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[Don A. Gilmore]

"Robert Scott" <no-one@dont-mail-me.com> wrote in message
news:42deba7b.37077808@news.provide.net...

Don,

I thought your self-tuning piano was further along than this. Or is
this a new project?


-Robert Scott
Ypsilanti, Michigan

Hi Robert:

Yeah, this is it. It has been on the back-burner for over a year while QRS
worked on another invention of mine (guitar-related). We are back in full
force on the piano now. I already have over-sized experimental sustaining
coils in the prototype, but now I need to trim them to a production-worthy
cost. So I'm going to experiment to see what the minimum field I need for
sustaining is, and then design the cheapest coil I can. Since there are
over 200 of them in a piano, I hope I can keep them under $1.

Don
Kansas City

 

[YD]

On 20 Jul 2005 09:44:03 -0700, "eromlignod" <eromlignod@aol.com>
wrote:

Hi guys:

I'm working on an application where I need a simple DC coil that
provides a particular magnetic field strength. I have to experiment to
determine just how strong the field needs to be. So I have wound an
arbitrary coil to experiment with. I will vary the voltage on it until
I experimentally achieve the necessary field in my application.

Now when I know that this coil is producing the right field, how can I
measure it? If I can get a measurment of the field intensity, I can
wheel-and-deal mathematically with different voltages, wire gauges and
dimensions of a coil so that I can design one to meet the field
requirement.

Is there a simple way to do this without expensive equipment? I
suppose I could take apart my arbitrary test coil and calculate what
the field must have been, working backwards, but is there a more
direct, emperical way? I want to be able to say, "My application
requires ______ teslas".

Don
Mechanical Engineer

If you know the turns and the coil and core dimensions, as well as the
core permeability, the calculation is pretty straightforward, giving
Tesla/A or something like it. Just read off the current when the
field's on the dot, this should give you enough data to calculate an
optimised coil. The formulas are out on the net somewhere if you don't
already have a handbook.

- YD.

--
Remove HAT if replying by mail.

 

[Robert Baer]

eromlignod wrote:

Hi guys:

I'm working on an application where I need a simple DC coil that
provides a particular magnetic field strength. I have to experiment to
determine just how strong the field needs to be. So I have wound an
arbitrary coil to experiment with. I will vary the voltage on it until
I experimentally achieve the necessary field in my application.

Now when I know that this coil is producing the right field, how can I
measure it? If I can get a measurment of the field intensity, I can
wheel-and-deal mathematically with different voltages, wire gauges and
dimensions of a coil so that I can design one to meet the field
requirement.

Is there a simple way to do this without expensive equipment? I
suppose I could take apart my arbitrary test coil and calculate what
the field must have been, working backwards, but is there a more
direct, emperical way? I want to be able to say, "My application
requires ______ teslas".

Don
Mechanical Engineer

1) It is the *current* that determines the magnetic field.

2) Look in a good physics textbook on electromagets and magnetic fields;
you will find equations to calculate the magnetic field in the center of
the coil.

 

[Meindert Sprang]

"Robert Baer" <robertbaer@earthlink.net> wrote in message
news:GQIDe.6142$dU3.5728@newsread2.news.pas.earthlink.net...

1) It is the *current* that determines the magnetic field.
2) Look in a good physics textbook on electromagets and magnetic fields;
you will find equations to calculate the magnetic field in the center of
the coil.

Precisely. And make the length of the coil equal or longer than it's
diameter. Did that once and I could almost calibrate my gaussmeter with it.

Meindert

 

[Robert Scott]

On Wed, 20 Jul 2005 23:22:55 GMT, "Don A. Gilmore"
<eromlignod@kc.rr.com> wrote:

....I already have over-sized experimental sustaining
coils in the prototype, but now I need to trim them to a production-worthy
cost. So I'm going to experiment to see what the minimum field I need for
sustaining is....

Almost any size coil can be made to sustain oscillation, depending on
what circuit is used for feedback, on how close you can position the
coils to the piano strings, and depending on what stability you want.
Since you are multiplexing many coils to one sustaining circuit, you
can afford to put more cost into the sustaining circuit if it means a
reduction in cost of the coil. But if the desgn of the sustaining
circuit is already cast in concrete, then I understand your search for
the lowest-cost compatible coil.


-Robert Scott
Ypsilanti, Michigan

 

[Robert Scott]

On 21 Jul 2005 06:52:55 -0700, "eromlignod" <eromlignod@aol.com>
wrote:

Robert Baer wrote:
1) It is the *current* that determines the magnetic field.


Of course. But it is much easier for me to measure and vary the
*voltage* in my experiment. What current is passing through my test
coil is immaterial to me if I empirically measure the field anyway.
And if I calculate the field, it is still simple to do using the
voltage.

I don't know why you are interested in the DC magnetic field strength
when your application (sustaining an oscillating piano string)
involves AC signals. Especially at the higher frequencies, the AC
impedance may become significant.


-Robert Scott
Ypsilanti, Michigan

 

[eromlignod]

Robert Baer wrote:

1) It is the *current* that determines the magnetic field.

Of course. But it is much easier for me to measure and vary the
*voltage* in my experiment. What current is passing through my test
coil is immaterial to me if I empirically measure the field anyway.
And if I calculate the field, it is still simple to do using the
voltage.

Don
Kansas City

 

[eromlignod]

Robert Scott wrote:

On 21 Jul 2005 06:52:55 -0700, "eromlignod" <eromlignod@aol.com
wrote:

Robert Baer wrote:
1) It is the *current* that determines the magnetic field.


Of course. But it is much easier for me to measure and vary the
*voltage* in my experiment. What current is passing through my test
coil is immaterial to me if I empirically measure the field anyway.
And if I calculate the field, it is still simple to do using the
voltage.

I don't know why you are interested in the DC magnetic field strength
when your application (sustaining an oscillating piano string)
involves AC signals. Especially at the higher frequencies, the AC
impedance may become significant.

I had a feeling this was coming. I neglected to mention that it was
low-frequency AC to avoid the inevitable admonishing pep talk about
impedance. The highest frequency that I will encounter is about 4000
Hz, the lowest is about 27 Hz.

Don
Kansas City

 

[eromlignod]

Robert Scott wrote:

On Wed, 20 Jul 2005 23:22:55 GMT, "Don A. Gilmore"
eromlignod@kc.rr.com> wrote:

....I already have over-sized experimental sustaining
coils in the prototype, but now I need to trim them to a production-worthy
cost. So I'm going to experiment to see what the minimum field I need for
sustaining is....

Almost any size coil can be made to sustain oscillation, depending on
what circuit is used for feedback, on how close you can position the
coils to the piano strings, and depending on what stability you want.
Since you are multiplexing many coils to one sustaining circuit, you
can afford to put more cost into the sustaining circuit if it means a
reduction in cost of the coil. But if the desgn of the sustaining
circuit is already cast in concrete, then I understand your search for
the lowest-cost compatible coil.

Well, here's my latest setup. The sustainers are all independent.
Each piano string passes just above a tiny optical fork sensor
(photo-interrupter). The coil is really just an electromagnet, like in
a relay. When activated, it pulls the steel string down into the beam
of the fork sensor. The fork sensor interrups the current to the coil.


So it acts sort of like a relay buzzer. Each time the string is pulled
into the beam, the switch turns the coil back off and releases the
magnetic pull on it. Then it bounces back out of the beam and the
cycle repeats.

My large coil takes enough current that I need an intermediate
transistor, since the little fork switch can't handle it alone. I
would like to reduce the power consumption of the coil so that I can
switch it with the sensor directly. Obviously, I would also like to
reduce the coil cost.

Don
Kansas City

 

[Mark Fergerson]

eromlignod wrote:

Robert Scott wrote:

On 21 Jul 2005 06:52:55 -0700, "eromlignod" <eromlignod@aol.com
wrote:

Robert Baer wrote:

1) It is the *current* that determines the magnetic field.

Of course. But it is much easier for me to measure and vary the
*voltage* in my experiment. What current is passing through my test
coil is immaterial to me if I empirically measure the field anyway.
And if I calculate the field, it is still simple to do using the
voltage.

I don't know why you are interested in the DC magnetic field strength
when your application (sustaining an oscillating piano string)
involves AC signals. Especially at the higher frequencies, the AC
impedance may become significant.

I had a feeling this was coming. I neglected to mention that it was
low-frequency AC to avoid the inevitable admonishing pep talk about
impedance. The highest frequency that I will encounter is about 4000
Hz, the lowest is about 27 Hz.

Yabbut, you said elsewhere that

1) there are over 200 coils, and

2) you aren't worried about the current.

ISTM you're gonna have to start worrying about "minimal"
impedance effects per coil adding up and affecting the common power
supply for all those coils. You don't want your piano sounding like
a motorboat with a drunken pilot.


Mark L. Fergerson