Magnets.....

[DR]

Hello group,

An artist customer called this afternoon to briefly explain a project he wants me involved in.

What I know so far is the project involves setting up an array of magnets such that a piece of steel can be suspended in air between them. This will be part of a sculpture. Size is one thing I don't know.

This is somewhat a research project because we don't know if it can practically be done. Apparently there's budget for a little bit of research.

I'd like to know if anyone here has experience with this sort of thing. Or possibly someone may know of a toy or scientific gadget that does this that we might use as a model for our version.

Any help will be appreciated...

[nheng]

You could do it with an electromagnet and a feedback mechanism like a photo sensor. The electromagnet causes lift when the object breaks a light beam and reduces lift when the light beam is cleared of the object. We're talking very small objects without getting into a big magnet and some hefty drive circuitry.

Actually, location within the magnetic field could be tracked from a greater distance from the object using a cheap laser, some optics and a little fancier photo sensor.

Perhaps someone else can offer a working example that they've seen.

Den

[jfsmith]

How big is this sculpture, dimensions, what is made of, weight, is it hollow?

If this is large, electro magnets will have to be used, and rather powerful ones.

Stability can be a major factor, cause a flux in the field and it may take off like a rocket or just drop to the floor.

Plus if this is more than a desk top toy, it may stop watches and other things from working. And maybe a pace maker hazard.

I tried something like this a couple years ago, a bird in flight, not very large, got complicated and then I put it aside. Maybe I will get it out and try to make it work.

Jerry

[Evan]

I built one back in the sixties as an electronics project. It worked well. The one I made was from a project in the old Popular Electronics. It used a photo sensor and a light source to control a simple amplifier that regulated power to an electromagnet. A variety of sensor mechanisms are possible and the advent of "super magnets" make quite good levitation distances possible. Just to make it clear, there is no way to orient a group of permanent magnets without active control to produce stable levitation in a non moving or non spinning object. It has been tried numerous times by numerous people. The problem is because magnetism falls off as the inverse cube power with distance which makes any such array unstable in the extreme.

Here is a link to a current model.

http://www.giftsandgadgetsonline.com/noname39.html


You might find this link interesting:

http://www.gumas.netfirms.com/hover_engine.htm

And this one:

http://www.1worldglobes.com/Specialt...berotating.htm

[This message has been edited by Evan (edited 06-08-2004).]

[Paul Alciatore]

This can work in one dimension where the suspended object is constrained on a track or shaft or in a tube. The object has a north pole facing down and there's a north pole on a magnet that's facing up, under it. They repel and it floats. Also works with both south poles of course.

In a real world situation, it's not possible to design a FIXED magnetic field that will act on either a magnet or a piece of magnetic material in such a way as to produce stable suspension in mid air. The main reason why is the fact that magnetic fields tend to obey the inverse square law and any confoguration will be in unstable equlibrium for a very short time before it will either rapidly rise to the magnet above it or drop to the magnet or other surface below it. I have kept this vague as many different configerations are possible. But none will be stable.

One of two things will have to be added to the situation to produce suspension: a constrictive geometry or an active feedback system that senses the position of the object and makes rapid changes in the magnetic field strength in the locations needed.

An example of constrictive geometry would be the one dimensional example I already mentioned or perhaps an object with a symetric opening in the bottom that is lined with magnets all having the same pole facing out. Think of an inverted cone or a tall cylinder. A matching base would fit very loosely into that opening and also have magnets with the same pole facing out. The like poles would repel in any possible orientation (it's symetric) and suspend it a small distance over the base. But it wouldn't be a very impressive sight as the suspension would be hidden.

Objects have been suspended in a magnetic field by employing positional sensing and feedback. A ball bearing could likely be suspended under a single electro magnet with such a system. You would need a powerful magnet, a sensing mechanism, and an amplifying system to power the magnet. Such an arrangement would be self centering and would not require any additional mechanism to prevent drift to the side.

Seems to me that I have also heard of aluminum being suspended above an AC powered electro magnet. That was ABOVE I said. The changing magnetic field induces a current in the aluminum and it acts to oppose the original field. Again, I am fairly certain it will work in one dimension (with a rail or tube to prevent sideways motion) but for 3D stability I don't know. Perhaps several such coils could be arranged around the center to keep it from drifting to the side.

However, if you want a floating object in a work of art, it might be better to use compressed air. You have seen the "all in the air stream" demonstration. It bounces a bit but is essentially stable. Other (but not all) symetrical shapes should also work: you will have to experiment.

Sounds like a fun project. Please post pictures if, no WHEN it happens.

Paul A.

[Tony]

here is the 'engine' for the floating globe:

http://www.gumas.netfirms.com/hover_engine.htm

and here's a twist.. alot smaller, but interesting:

http://iml.umkc.edu/physics/sps/proj...ev/maglev.html

-tony

[Evan]

Paul,

Inverse cube law for magnets, not inverse square law. Also, as far as I know there is no geometry that will produce stable clear otherwise unsupported or constrained levitation using only permanent magnets no matter what the configuration, hidden or not.

Added: Superconductors notwithstanding. In a superconductor the magnet is "pinned" to the flux lines. The example in the link above is an exception but can only be achieved under extremely difficult circumstances. They didn't mention how long it took to get the setup just right.


I have seen an AC powered levitator for aluminum plates. GE made one for demo purposes and I saw it at the Chicago Museum of Science and Industry. It would lift a plate the size of a pizza plate about 4-6 inches, perfectly stable. It relies on eddy currents in the aluminum and gets hot fast! The unit had about a 1% duty cycle, one minute on and 1 1/2 hours off.




[This message has been edited by Evan (edited 06-08-2004).]

[Cass]

SKF among others has built some magnetic bearing spindles for machine tools that work. The spindle floats in the journal with no contact at zero rpm and at speed. Forest-Line a french company used a magnetic bearing spindle on a "high speed" machining center that they showed at the NMTBA show in Chicago. "High speed" machining means a one inch endmill going 40,000 rpm cutting aluminum. Fairly common these days in machining aircraft parts; it was exciting to see it then which was more than 15 yr. ago. Complex circuitry for the spindle. You might search for magnetic spindles and SKF and get some insight about how hard it is to do what you want. I think you will find it is not worth the expense and time.

[hms50]

Here is a site that may be what you are looking for: http://www.scitoys.com/

Good Luck

hms50

[DR]

Hey, thanks guys.

I'll be meeting with my customer today. I forgot to mention he has already purchased a bunch of small magnets. Apparently he's hoping to do this project without any sensing devices or electromagnets. My experience playing with toy magnets told me this may not be possible.

I'm printing out your responses and links to take with me. I'm sure they'll either kill the project or send it off in a slightly different direction.

Thanks again. I'll let you know how things turn out.

[ibewgypsie]

I worked with a small vibratory motor using magnets and electromagnets.

The previous items (tattoo machines) have a spring. By pulsing polarity from positive to negative you can operate the tattoo machine at a much higher frequency then possible with springs.

Thought? the faster you can tattoo the more money you can make.

David

[nheng]

Paul said: "This can work in one dimension"

Mall gadget stores frequently have a small dumbell shaped piece suspended horizontally between multiple magnets and restrained by a mirror at one end (location of the magnets causes a slight force toward the mirror but the mirror friction is small). It is cute.

The whole thing is only about 6 inches long though but could be scaled up. No power needed. Let the wind blow thru it somehow and you'd have random rotation as it floats.
Den

[mayfieldtm]

I used to work for a custom magnetics company.
We would get this type of request on a regular basis from Artists and Inventors.
And don't ask about how many perpetual motion machine designs I've seen.
If he can live with a mono-filament attached to the bottom of his piece to keep it steady, and use a LARGE deep field magnet above, that would work.
Levitation with electro magnets and position sensors as mentioned above, will work, but, typically for small items only.
And also mentioned, Levitation of Aluminum objects works great using Alternating Current in a coil. (One coil only) This works best on thin material with large surface area like a disk or foil. Needs hefty currents.

Tom M.

[Paul Alciatore]

<font face="Verdana, Arial" size="2">Originally posted by Evan:
Paul,

Inverse cube law for magnets, not inverse square law. Also, as far as I know there is no geometry that will produce stable clear otherwise unsupported or constrained levitation using only permanent magnets no matter what the configuration, hidden or not.
</font>

Evan, Electromagnetic forces in general will obey the inverse square law. In the case of real world magnets, which always have two poles, things get a lot more complicated and the actual increase or decrease in force will need to be found taking all factors into account. In some situations it will be close to inverse cube, in others closer to inverse square. Neither will ever be exact.

As for the geometry I envisioned:



Something like this should float. Kinda like magnetic bearings.

Paul A.

[This message has been edited by Paul Alciatore (edited 06-08-2004).]

[Evan]

Paul,

Magnetism is the odd one out and obeys inverse cube law.

http://image.gsfc.nasa.gov/poetry/activity/l13.htm

Paul, I know it seems that something like that should work but it won't. The poles of a magnet are always "connected" by magnetic flux lines no matter how they are arranged. There is no such thing as a monopole and the north and south poles of the magnets on both constructions are "visible" to each other.

[This message has been edited by Evan (edited 06-08-2004).]