Practicallity of maglev table?

On: Tue, Jun 26, 01 05:06:35 PM

Steve wrote:

RESPONSES

Colin - Tue, Jun 26, 01 10:12:53 PM

How much weight are you going to support? 64.170.192.169

Ray - Tue, Jun 26, 01 11:30:33 PM

Wouldn't the magnets need to be electromagnets? This would be more efficiant. 209.179.148.209

blueeyedpop - Thu, Jun 28, 01 10:23:51 AM

It couldn't hurt to try... The difficulty I see is trying to get the magnets configured in such a way as to limit lateral shifting, so the table doesn't "fall off" of the magnets. Also, any forces on the magnets will still be transmitted to the table via magnetic forces. 64.63.88.51

Colin - Fri, Jun 29, 01 12:27:55 AM

blueeyedpop, Aren't the magnetic forces a perfect spring? Air has long been used and it is also a perfect spring. 63.193.192.224

blueeyedpop - Fri, Jun 29, 01 02:24:40 AM

Hmmmmmmmmm. But have you ever ridden in a car with really bad shocks? Very bouncy. I suppose it would eliminate a lot of high freq. stuff. 64.63.88.51

Colin - Fri, Jun 29, 01 06:57:06 AM

Yes, there is springing, damping and isolation to be considered. Isolation comes from creating an impeadance mismatch between the ground vibrations (3hz to 10hz) and the table resonant frequencies (If my musical instrument design experience translates correctly). My new table should have good isolationn down to 15hz. Below that I only have damping. What a air isolation table and a magnetic isolation table would have in common is only vertical motion would be transfered to the table! The horizontal motion would go unnoticed until you reach the stops that keep the table from sliding off. 63.193.192.117

Steve - Wed, Jul 04, 01 03:15:58 PM

I've realised a new problem In order to stop the table sliding off the magnets, I was going to use a ring/rod magnet combination, where the rods would be a high gauge than the hole in the rings, and similar poles. I came across 2 problems very quickly, 1. the rods cannot have the same pole all the way around! I should have thought of that right away, second, since the magnets now have a common horizontal plane, any horizontal vibrations are transmitted through them. Damn Catch 22. As far as I can tell, there is no other way of isolating the magnetic planes, any non-magnetic form of containment of the upper half of the table would defeat the entire purpose by transmitting vibrations. The other option, would be far more expensive, but prehaps using a suspension system? Powerful magnets mounted on the ceiling, etc. give lift to the table hovering below. The problem with this is any vibrations in the ceiling, which would be most likely very prominent, from wind and such, would be directly carried through the magnets into the table, mit would be nearly impossible to put the supporting magnets into isolation. Back to the drawing board. 142.32.208.231

Michael - Thu, Jul 05, 01 06:08:55 PM

I am very new to holography making but very interested. It seems reasonable to assume that the magnetic setup to issolate vibrations vertically could also be used on the edges to prevent the table from sliding to far in any of the four directions. This would increase your magnets from one to four however and I have no idea of their price. 12.18.158.25

Colin - Thu, Jul 05, 01 10:23:00 PM

The horizontal bumpers could be made of sorbothane or the magnets could be designed with a "well" in the center to hold the table from falling off. A magnet above a circular electro-magnet springs to mind. 64.170.192.77

Steve - Sun, Jul 08, 01 12:27:59 AM

Colin - Sun, Jul 08, 01 01:36:59 PM

Or think of this top view: nn nn nn nn s nn nn nn nn And this side view: n s nnnnnnnnn sssssssss Then the center magnet has to lift to get out of the center. My guess is this would couple less of the horizontal motion to the system. 63.193.192.193

Colin - Sun, Jul 08, 01 06:04:53 PM

Woops! Try reversing the top magnet. 64.167.150.171

Steve - Sun, Jul 08, 01 07:42:33 PM

Hmm... The only problem I can see with that would be that theres no way to tell what any problems could be without commiting yourself to buying the magnets and checking. Unless anyone knows of any formulas to calculate magnet fields and more important, interactions between magnetic fields. 142.32.208.231

Colin - Mon, Jul 09, 01 01:02:11 AM

I have a toy that has this magnet configuration. It is stable in the center then it falls off when it exceeds a critical distance from the center. http://www.treasure-troves.com/physics/ElectricField.html shows the mathamatical development of electric field theory. A high school physics text would give a easier to follow explanation. I don't seem to have kept mine. It would be very easy to make a small model to check your work. Say a table 1'x1' for single beam reflection work. You could make the base magnet an electro magnet to save money. 64.167.148.29

Steve - Mon, Jul 09, 01 02:28:10 AM

Please read my original posting, I am only just beginning to get into holography and haven't constructed my table yet, I have a very limited space so I need to be able to pack this away, but I want the size and stability of a larger table, thats why I was looking into maglev. I don't want to begin spending money until I am sure I can have a finished working product. And being that I have only just finished Physics 11, my head starts spinning at the sight of the complex equations I've found so far regarding magnetic fields acting on particles, nevermind magnetic fields acting on other magnetic fields. 142.32.208.231

Colin - Mon, Jul 09, 01 11:42:14 AM

Steve, Any time you are on the cutting edge plan to get burned. If you want to design a new type of table it will be much easier if you have some experience with holography first. My table is unconventional and I have had to do a lot of tweaking to get it to work. It has cost lots of film and time. Every time I make a change I have to break down the set up and make an interferometer and test things out for a few hours. My latest three exposures show the object was in motion :-( When I was designing gliders (I never built any) my flight instructors told me "If you want to fly, fly. If you want to build, build." If you want to do holography copy someone's table first. When you understand all of the other variables then design a new kind of table. But if your like me and the design and debugging is half the fun expect to spend time and money on things that sometime fail. Just my two cents. You say you want the size and stability of a larger table. The truth is the stability comes from stiffness (High resonance frequencies are easier to damp out). The larger you make your table the harder ($$) it is to make it stiff. Effective isolation is difficult to achieve with any means. I have been working with the engineers at sorbothane for a few weeks on converting my table from inner tubes to sorbothane. My inner tubes only worked at a very specific pressure range and I could not fill them without lifting my table off them. They would change pressure every couple of weeks. Sorbothane appears to cost $2.50 for every 15 pounds. So the cost adds up very fast for a large table. This could be cheaper if I made some 1.5" thick donuts but sorbothane is not very easy to cut so I am using precast hemispheres. If you would like some help on a specific design e-mail me at colinsk@pacbell.net and let me know what size table and what kind of holograms you want to make and we can discuss the pros and cons of the available options. For other points of view try looking into the older posts. There is a post from a researcher who did a study saying that attached to the ground was better than most isolation systems. Blueeyedpop never ceases to amaze me on what he is able to do with his 2' x 2' table! He is very inventive! 64.167.148.29

Steve - Mon, Jul 09, 01 04:00:43 PM

Thats another thing I've been thinking about, I live way out in the country, do I even need to worry about anything beyond very basic isolation? As I said, I am new to the field, but I've never liked following in other peoples footsteps, I like to take things to the next level, or at least an unexplored parallel level. 142.32.208.231

Frank DeFreitas - Mon, Jul 09, 01 11:15:09 PM

Interesting discussion. A variable to consider (and which is not often considered) with vibration isolation is time -- and how a shock wave moves through 3-dimensional space. Theoretically, a hologram could be created on a table that is violently shaking -- providing that everything on the table is shaking in unison. It is only when one area of the table is in motion, relative to another area, that fringe recording becomes impossible. With both ground and air vibrations, the source of vibration moves through space. If a truck goes by your lab, the vibrational wave travels from the source (street) to your table. It hits an area of your table closest to the source first, and then continues to travel as it passes underneath, until it is gone. This creates a vibrational problem at one end of the table (at a specific point in time), compared to the area that has not been effected yet (future). Granted, this happens within a relatively short amount of time (depending on the mass that the vibration is traveling through, i.e. dirt, rock, etc.) -- but without isolation will ruin the exposure. So, in essence, what we're looking at is setting up a system that depends more on stiffness -- reducing flex, even at micrometer-level measurements. A system that would exhibit "infinate" stiffness would not need to be isolated at all -- since the entire system would vibrate in harmony with the shock wave, retaining fringe integrity. This is why John Perry's system at Holographics North is embedded right into bedrock. It's the bedrock that is providing the "stiffness" of the system. Work on reducing table flex as the primary objective. Then design the best vibration-isolation around that. Frank 208.59.249.8

Steve - Tue, Jul 10, 01 01:00:38 AM

I am EXTREMELY annoyed. I just finished an extremely long post and accidently hit Erase the Form instead of Record Response, Frank, would you mind adding an 'are you sure' option? That would be very nice. Anyway, I was proposing an idea for testing the rigidity of the table. As bolting directly into the bedrock would defeat the whole purpose of portability, I was suggesting a multi-step isolation set up, starting up with this: Set up an inferometer on your table, find a large subwoofer (I'm guessing it would neeed to be at least 8", it doesn't need to be quality, you can probably get it for under $40) and burn a cd that has digital square waves advancing 1hz every 5 seconds, 1hz-2hz-3hz, etc. Aim the woofer at the TOP of your table, crank it and time when the inferometer starts going off, thats your golden number. Anything from Xhz and up needs to be isolated, below Xhz can be ignored. Any suggestions/concerns? 142.32.208.231

Colin - Tue, Jul 10, 01 06:46:33 PM

I am happy when I can walk by my table with out upsetting it! Oh yea, Use a sine wave. A square wave has harmonics. More traditional is to place a sensor in a hammer record the signal from the hammer and a sensor placed on the resonant object. Strike the object softly and perform a FFT on the data from the sensor. It generates a very nice impeadance curve in under 1 sec. 64.167.151.221

Colin - Tue, Jul 10, 01 10:36:37 PM

Frank brings up a good point. If all of the optics and the object stay in position relative within roughly 1/10 wave length of each other an exposure can be made. I have as many problems with my optic mounts as my table in this regard. I would imagine my optic mounts would need to be mounted on 2" dia shafts to remain stiff enough to be shook with any violence. My 1/2" steel mounts are too flexible. 64.170.192.128

Steve - Wed, Jul 11, 01 12:35:34 AM

1/10th of a wavelength.. Thats not very far... :D Hmm, what about using a high power laser, such as Frank's 25mw diode, on a relatively small plate, such as 4x5 and use an SLR camera suspended from a tripod with the back cover and lens removed, so only the shutter was in the way of the beam, it would be possible to have extremely short exposures, so stability would be less of a problem, I don't suppose there is any 'minimum' exposure time, as long as the plate gets enough light, by that I mean no overall minimum, of course there is a minimum for the specific set up... 142.32.208.231

Colin - Wed, Jul 11, 01 02:09:20 AM

Pulsed lasers are used all of the time with 30ns exposures. There is no need to mount the shutter and laser to the optics bench. I am not sure why this works but many holographers use this method. The best shutter I have encountered is a vu meter with a flag on the needle to block the beam. No vibrations. If you want a mechanical shutter look into the copal shutters for enlargers and large format photography. 64.170.192.128

John K - Wed, Jul 11, 01 09:23:58 PM

If you don't need to keep the laser or shutter on the optics bench to isolate it from vibration. (That's how I have been doing it) Why can't you opticaly form the beam into a thin line and scan the object and plate? It would provide a brighter beam than expanding it,and the plate would only be exposed where the thin beam was hitting the plate, providing a short exposure. I know that copies of holograms can be done that way, but the saxby book says this isn't suitable for making a regular exposure. There must be a reason why, but I haven't seen an explaination of why it can't be done. 63.228.11.191

Colin - Wed, Jul 11, 01 11:15:58 PM

The problem would be from the object beam. Even though you light the object in a line the reflection disperses to the entire plate. The area of the plate that has the reference beam would still form fringes but the rest of the plate would be getting exposed without any fringes. When the line got down to the already exposed area some of the silver would be just noise. You might be able to make a difraction grating this way. Or you could move a mask down the plate so only the area around the line is illuminated by the objest beam. This sounds like it elliminates the simplicity we are searching for. 64.167.151.174

Steve - Thu, Jul 12, 01 01:19:53 AM

Hummm.. Any vibrations smaller than 5hz would have a maximum velocity of 277 m/s, so, If your table is under 5m long, an exposure of 0.018s would nihilate any vibrations. Would someone double check my calculations? Also, anyone know where I can get a table that says how many seconds an exposure will take on a certain plate size with a certain power of laser? Thanks! I think I'm going to work on a self-contained hologram 'machine' i.e. Put object in light-proof box, press the button and the laser does all the work, with a fast enough shutter, you should be able to virtually eliminate any interference caused by vibrations.... Ideas? 142.32.208.231

Colin - Thu, Jul 12, 01 01:43:01 AM

You can find exposure information on the slavich web site for their emoltions. I measure my exposures in seconds with a 27 mw laser for pfg-01. My shutter is a black card I remove from the beam. 64.167.151.174

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