Magnetic anti-vibration platform with diamagnetic stabilizers - diyAudio
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Old 10th May 2003, 07:18 PM   #1
Prune is offline Prune  Canada
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Default Magnetic anti-vibration platform with diamagnetic stabilizers

There have been previous threads here and at other DIY boards about magnetic levitation anti-vibration platforms for CD players and DACs, as well as a few commercial products. The thing is, all of these designs use a physical constraint to achieve stability, thus limiting the dampening mostly to the vertical dimension.
Of course, Earnshaw's theorem shows that it is physically impossible to achieve stable levitation with permanent magnets in a static field. Either there are physical constraints (bad -- carry vibration), or the magnets are in motion (like the levitating spinning top toy; bad again -- some vibration), or the field is not static (bad yet again -- active field will induce some vibration).
Another approach is to use diamagnetic materials for stabilizers.

Unfortunately, the best diamagnetic is a superconductor and that's not practical. Bismuth is the best room temperature diamagnetic, but the effect is still very weak. Still, with neodymium-nickel-boron magnets visible passive levitation can be achieved.

It might be possible to use the following design: have a platform with magnets at the corners repelling from magnets at the base, just as in the commercial products, but instead of physical constraints, have the platform magnets in a vertical bismuth tube. The diamagnetic tube should repel the small magnet enough to keep it from touching it, and prevent the platform from moving out of equilibrium. The problem is that the platform would have to be light for this to work, and thus its magnets small, and the base magnets very large, possibly intefering with electronics on the platform.

Anyway, I ordered some N45 grade NdFeB magnets from eBay, along with some bismuth to mess around with this idea. Bismuth can be melted on a gas stove, so it shouldn't be too hard making tubes...
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Old 10th May 2003, 08:07 PM   #2
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Quote:
Of course, Earnshaw's theorem shows that it is physically impossible to achieve stable levitation with permanent magnets in a static field. Either there are physical constraints (bad -- carry vibration), or the magnets are in motion (like the levitating spinning top toy; bad again -- some vibration), or the field is not static (bad yet again -- active field will induce some vibration).
Why limit yourself to passive? The field can be controlled.
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Old 10th May 2003, 08:39 PM   #3
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Because you cannot control the field perfectly smoothly, and there will be some variation in it, which means your platform won't be completely static after all. Perhaps you could tune the controller so that any oscillations of the platform will be in a frequency range that would not affect the device being isolated...
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Old 10th May 2003, 09:10 PM   #4
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You can't perfectly isolate passively. either. If you're after perfection, better run your equipment in a vacuum so that acoustic waves can't impinge on them. Besides which, "active" doesn't mean "absence of passive means." All an active system needs to do is stabilize a passive system that's inherently unstable. Active control methods are good enough to get an F117A to fly smoothly; suspending a stereo system ought to be a cakewalk, so to speak.

Mmmmm, fuzzy logic controllers....
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Old 10th May 2003, 10:01 PM   #5
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What such a system does is isolating vibration between the isolated platform and the construct it is resting on. It does not damp resonances in the platform itself.

It is basically a mass spring system. Two magnets repelling each other are simply a spring. It differs from a normal steel spring in that the force is quadratic inverse with distance (in the ideal case without magnetic leakage) whereas with a normal spring force is linear inverse with distance.

Driving such a system active can increase the compliance of such a system and as such lowering its resonant frequency. Also damping can be controlled dynamically, although I see no use for it.

I donít see any advantages of such a magnetic system over a normal spring system in case of audio equipment. It is only more complex and expensive.

Both systems need proper damping.
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Old 11th May 2003, 12:18 AM   #6
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donít see any advantages of such a magnetic system over a normal spring system in case of audio equipment. It is only more complex and expensive.
That's true of a lot (most?) things in hobby audio. That's the part that I understand, a frightening thought. The coolness factor: diamagnetism! bismuth! neodymium! Even if there's no real problem to solve, it's so cool, it's worth doing just for the sake of doing it.
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Old 11th May 2003, 12:47 AM   #7
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Quote:
Originally posted by SY
That's true of a lot (most?) things in hobby audio. That's the part that I understand, a frightening thought. The coolness factor: diamagnetism! bismuth! neodymium! Even if there's no real problem to solve, it's so cool, it's worth doing just for the sake of doing it.
Kind of like blue LEDs. I mean, a blue LED doesn't impart any more information than say a red or a green LED. But blue just looks soooooooo cool. Hovland perhaps having taken this to utter extremes.

se
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Old 11th May 2003, 01:27 AM   #8
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Originally posted by SY

That's true of a lot (most?) things in hobby audio.

And as usual, the ABX DBT won't show a difference...
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Old 11th May 2003, 06:00 AM   #9
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No, probably not. But that's not why we do what we do.
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Old 11th May 2003, 11:25 AM   #10
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Originally posted by SY

Even if there's no real problem to solve, it's so cool, it's worth doing just for the sake of doing it.

True SY and that is the main part of he fun Long time ago I designed a magnetic bearing for my turntable to hold the heavy platter: No friction, no wear. A proper designed magnetic bearing can hold 100N/cm^2 as a rule of the thump. It was fun to work out the idea but actually implementing it was many miles too far

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