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Shielding Generator

Posted: Tue Jan 30, 2007 9:44 am
by bdickens
I've run into a snag with my water monitors. The generator (a DC motor) is working fine and producing a reasonable amount of current (although anyone who knows of a micro-generator less than 3 inches long, email me).

But in my test rig, it's making my compass read incorrectly.There will be only about 8 inches seperating them. Am I correct in assuming that if I put a wire cage around the generator, that the magnetic field will be reduced to the point where I won't create false changes in direction ?

Posted: Tue Jan 30, 2007 10:11 am
by Chris Smith
You will have to experiment with the magnetic cage and strength.

A tiny generator from almost any DC toy motor will generate small amounts of voltage and current, possibly as much as you already have.

Posted: Tue Jan 30, 2007 10:44 am
by rshayes
It sounds like the permanent magnet in the motor (that you are using as a generator) is interfering with a magnetic compass that you are using to detect direction. This is not suprising, since the field within the motor is three or four orders of magnitude stronger than the earths magnetic field. Most of the field will be contained in the motor housing, but a significant amount leaks into the surrounding area.

One way to alleviate this is to locate the compass in a region where the leakage field is either small, or in a direction that will not affect the compass reading. A field passing through the compass in a vertical direction will have much less effect, since a compass should be mainly sensitive to the horizontal component of the earth's magnetic field. The best position for the motor is probably directly above or below the compass with the motor poles also on a vertical axis.

The motor can also be shielded by surrounding it with a cylinder of magnetically "soft" material (a material that is easy to magnetize but does not retain magnetization). There are special materials made for this, but they tend to be expensive and hard to form to shape. Thicker sections of less ideal material might work. Thin layers of iron, separated by nonmagnetic material, might be more effective than one thick layer.

These two technques, combined with the maximum separation that you can get between the motor and compass, might be enough to allow reliable direction readings.

Posted: Tue Jan 30, 2007 11:00 am
by haklesup
Not just any metal and not a cage, a solid barrier is required here. Your reasoning (wire cage) better applies to electric fields.

Mu-Metal is one of the best "magnetic shielding materials" but a search on that term also turned up flexible polymer materials as well. You can usually get a small sample for free. Mu-Metal has been around a while and you may have an old chassis or power supply box made of it. Another source would be some "shielded" audio speakers. I've ocassionally seen a spiral of magnetic shielding material sprung around the speaker magnet

Even if you cannot shield all of the magnetic field of the motor, if you can get it low enough, many electronic compasses have some means for canceling static fields. I've owned several car models and they all had some sort of calibration procedure where you drive in circles while it detects the static component of the field and ignores it. Does your's have this capability.

Posted: Tue Jan 30, 2007 11:40 am
by rshayes
Mu-Metal is the oldie but goodie for magnetic shielding purposes. It was used for shielding CRTs in radar displays during World War II and after. It has the unfortunate characteristic of changing its magnetic properties when it is formed. The magnetic properties are usually restored by an annealing process in a furnace with a hydrogen atmosphere. If you can get someting already formed, this would be the best choice.

Some of the MetGlas materials are also sold for magnetic shielding. This is usually in tape form. The properties do not change as much as Mu-Metal when the tape is formed, so this is a reasonable approach. The disadvantage is that the tape is quite hard, difficult to cut and bend, and it has incredibly sharp edges. This may also be expensive and hard to get in small quantities.

Shielding Generator

Posted: Tue Jan 30, 2007 2:25 pm
by bdickens
Ahh. Interesting. Ok, I'll see what I can find on those topics. As always, thanks.

Posted: Tue Jan 30, 2007 3:36 pm
by jollyrgr
Correct me if I’m wrong but don’t you need a non ferrous metal like copper, brass, or aluminum for such a shield? This won’t be like the metal box used to shield a tuner in a receiver. And a screen is more for RF instead of magnetic.

Posted: Tue Jan 30, 2007 4:24 pm
by MrAl
Hi there,

Another interesting idea is to use dynamic cancellation.

Since the motor wont be generating too much of a field even
eight inches away, a small coil with magnetic polarity opposite
that of the motor should be able to generate enough of a field
to cancel all of the motors EMI. Although a small sense
coil might work to sense this EMI intensity and direction, a better sensor
for this would be a hall effect device (the linear type which senses fields
in both directions) because it can sense a possible dc offset field
as well as the normal changing part. Of course you could try a
coil first.
The idea would be to use it with a cheap feedback amp and to
have the amp drive the main coil (both plus and minus) until the
net field four or five inches away is zero. An NPN driver would
increase the current capability of the amp, connected as voltage
follower, so you can use a run of the mill op amp.

What else is interesting is that from what you are saying the motor does
have a static dc field coming from it. I have to wonder if you cant just
rotate the motor to see if the field from another angle will net to zero.
Of course you'll have to be able to rotate the motor around the three
3d space axes to try this trick.

Since it seems that some orientations might produce a dynamic field
where each polarity of the field is of equal intensity, that might not
bother the compass except cause it to possibly oscillate a little, and
maybe the static part can be canceled out with a small magnet placed
somewhere between compass and'll have to try this.

Posted: Tue Jan 30, 2007 4:56 pm
by Chris Smith
We used to plate the insides of equipment so that EMI wouldnt leak out, but the copper mix was also non magnetic.

The farady cage does apply, but only if the whole material is non magnetic.

Posted: Tue Jan 30, 2007 8:41 pm
by Newz2000
The title to this thread gave me flashbacks to the beginning of "Empire Strikes Back." Wasn't DV trying to find the rebel base and take out its shield generator?

Posted: Tue Jan 30, 2007 11:02 pm
by dyarker
To shield from a magnetic field you need a magnetic material, like soft steel. To shield for an electric field you need a conductor, like copper. Steel being conductive can be a shield for both, but copper plated steel would be better. Coverage is key. Screen is okay as long as holes are smaller than thickness of screen. (This is frequency dependent, but you said DC.) ((Conditioned air is often fed into "screen rooms" through metal "honey combs". The openings in the honey comb block transmission of fields due to a wave guide effect. The ratio of largest crosssection of openings to length determines highest frequency blocked.))

Another source of interference is the wires bringing power from the motor being used as a generator. Current flow creates both E and M fields. Making a twisted pair of these wires will help some. Routing them as far away as possible from the compass unit will help some more. Distance and orientation help as the others have said.

Hope this helps. Cheers,

Posted: Wed Jan 31, 2007 8:28 am
by Chris Smith
Our plating place added in either or both the anti RFI and/ or Magnetic shielding layers into medical equipment.

The copper mix was good for RFI while the Nickel mix was good for the magnetic alterations or control.

From the web….

There is no known material that blocks magnetic fields without itself being attracted to the magnetic force. Magnetic fields can only be redirected, not created or removed. To do this, high-permeability shielding alloys are used. The magnetic field lines are strongly attracted into the shielding material.

Posted: Wed Jan 31, 2007 11:57 am
by haklesup
Stainless steel (which is high in nickel) seems to be a pretty good shield for both magnetic (magnets don't stick to it) and electric fields (its conductive).

A simple component might be a stainless steel shot glass or drinking cup (see toothbrush cups also). These are relatively easy to find and are approximately the right size and shape. Not perfect but coupled with a nother magnet placed elsewhere to cancel out the remaining static field from the motor magnets might be enough.

For a DC near-Field Magnetic field only a solid barrior which can redirect or absorm the magnetic field lines will work. All that stuff about holes and screens only applies to far field EM waves

You no doubt have a setup to check its effectiveness. And of course you could abandon the magnetic compass in favor of an absolute pointing method like shaft encoding, though this would require more complex anchoring.

BTW, for measuring clarity, I had one of these optical sensors in mind Just to be clear

Posted: Wed Jan 31, 2007 12:55 pm
by ecerfoglio
Stainless steel (which is high in nickel) seems to be a pretty good shield for both magnetic (magnets don't stick to it) and electric fields (its conductive). (haklesup)
Non magnetic materials do not shield magnetic fields. You need a magnetic material.

In fact, some submersible deep well pumps are made with a stainless steel (non magnetic) casing between the motor's windings and the rotor. The rotor is immersed in water and the windings are kept dry by the stainless steel "wall"

To eliminate the effects of an static field (the motor's own field magnets) you could do the following:

>> put a pair of magnets outside the motor so their fields oppose the stray fields of the motor,
>> enclose the motor (and the extra magnets) in a piece of iron or steel. A length of thick walled pipe (with an inside diameter slightly bigger than the motor plus the magnets) should work.
>> rotate the motor (in fact the entire assembly: motor, magnets, pipe) arround its axis so the resulting stray field is vertical (you want to measure the horizontal component of the Earth's field)

this way the stray magnetic field of the motor gets neutralized by the magnet and any difference follows the "path of less resistance" around the pipe.

You can evaluate the effect of the different "countermeasures" with the compass, trying different magnets (and their orientation) to find the best combination.

Posted: Wed Jan 31, 2007 3:16 pm
by jollyrgr
Newz2000 wrote:The title to this thread gave me flashbacks to the beginning of "Empire Strikes Back." Wasn't DV trying to find the rebel base and take out its shield generator?
In a way the rebel's shield generator was their undoing. Smugglers, slave traders, and so on would never need such a huge shield generator. It was a dead give away to Vader that this was no small time operation. I still find fault with the fact that they used AT ATs to walk to the shield generator. If they can land the AT ATs on the the ground and have them walk to the generator why not land TIE fighters or other flying craft in the same location and simply FLY to the generator?!?!