Aluminum Can Tables - what's new?

[Guest]

John, have you thought about hanging the table from four cables attached at each corner of the table to a single eye hook in the ceiling?

That still has similar vibrational problems unless the ceiling is somehow isolated from the rest of the structure. The vibrations coming into your area don't all travel through the floor.

Aside from the "vibrations" coupled through the ceiling, wouldn't this design make a nice pendulum...that can swing in 2 dimensions?

[JohnFP]

Well, provided you did a study on what the frequencies of the vibrations were that were reaching the table, you could differentiate the lengths of the cables to cancel those frequencies out. Be careful, you could cause amplification!!!

[Dinesh]

[quote:e27ac5ab87]Well, provided you did a study on what the frequencies of the vibrations were that were reaching the table, you could differentiate the lengths of the cables to cancel those frequencies out.[/quote:e27ac5ab87]
Not so. The frequency response of a cable with moment of inertia, I, is:
T = (2*pi)(sqr root(I/g))
You can see that, therefore omega is inversely proportional the moment of inertia. You'd need to calculate the Fourier components of the frequencies and adjust for a multiple number of different moments of inertia. I'm not sure this is possible. Even if it were, the difference in thermal properties would have a greater effect than any supression of varying omega factors.

[quote:e27ac5ab87]Be careful, you could cause amplification!!!
[/quote:e27ac5ab87]
I assume you mean resonance. If the driving frequency is omega_d and the natural frequency is omega_0, the resonance occurs when,
omega_d ~ omega_0.
This can be seen from the amplitude,
A = (F/m)/(omega_0^2 - omega_d^2)
Clearly there is an asymptote at the resonance condition given above. However, for real-world conditions, this again needs to be modified to:
S(omega_di) - omega_0
where I've used S as a capital sigma. The various omega_di would correspond to the Fourier components of the driving frequency.

[BobH]

Regardless of the physics of the stability of a hanging table, can you imagine actually working on such a structure!? We don't see tables hanging from the ceiling for good reason. C'mon people, can't we think a bit before typing out half-baked ideas or mathematical gobbledygook?

[Dinesh]

[quote:2a03bf508d]C'mon people, can't we think a bit before typing out half-baked ideas ormathematical gobbledygook?[/quote:2a03bf508d]
I completely agree! I've been saying for quite a while now that not every idea/concept should be posted simply to show the cleverness of the poster. There are clearly ridiculous ideas such hanging tables or heating large blocks of metal with IR from the sun which are just too silly to seriously think about. The problem is that there is a Law of Large numbers which basically says that if every idea or concept - the useful and reasonable along with the rubbish - is posted, then very quickly we'll be drowned in rubbish. I think the response that all ideas are valid and no one should set themselves to judge them or they're trying to "control" the forum is basically flawed. Common sense is a great indicator of rubbish. As you say, Bob, who in their right mind would even think of hanging a table (except in idle fantasasing!)
Problem is: what d'you do? ignore it and let The Law of Large Numbers take over? . Just say, "Don't be silly!". This is like a red rag to a bull! i thought that if I patiently explain why this is silly, and throw in lots of mathematical goobledygook, then the originators of such ideas then have to answer the mathematics rather than continuously pile strained explanations on top of one another.
Sorry, but I've seen far too many "perpetual motion" types and other crazy ideas take over in the past few years. A lot of them seem to gain legitimacy by using semi-technical jargon that I know they don't understand or throw-away phrases such as "Be careful, you could cause amplification!!!" which are actually meaningless but makes it seem that there are subtleties that most of us don't see in a complex problem . If no one disputes the silliness of these ideas, then they end up being seriously considered. I usually try to explain anything in as non-mathematical a way as possible. Except to deflate

[JohnFP]

Bob, actually I can envision working on exactly such a table if it conquered some additional stability problems.

Had the thought been taken with an open mind you would see many hurdles could be over taken with positive thought.

1. For additional dampening, the four cables would be attached to a rectangular support, which would hold some inner tubes which in turn would hold the holographic table. I feel this table would dampen better then a table on a solid floor. Just intuition tells me there would be a degree of additional dampening aquired from four thin cables, maybe even Kevlar cord, as oposed to large cinder blocks. The cables could even have rubber rings on them to dampen vibrations like a loose finger on a guitar string. Remember, if the table moves wholey, there is not problem.
2. Maybe the four cables attached at the ceiling are attached to a flat rubber object, like the polynorbornene "sad ball".
3. Just because the table is hanging, it does not mean that table has to hang freely while setting up your table geometry. Four support/stabilizing legs could come up and rigid the table while configuring he table. Then release the legs and allow the table to hang and settle.

This is exactly the type of "dismiss an idea" destructive response instead of "how would you over come this hurdle" or "how about if you did this to control this" constructive response I am talking about.

You know, holographers are by nature a very creative, brilliant, ingenuitive, skillful breed. If they learned to be positive and helpful to one another I am sure holography would find the avenue to success it has been looking for.

Peace!

[BobH]

If they learned to be positive and helpful to one another I am sure holography would find the avenue to success it has been looking for.

John, I respect your enthusiasm, obvious intelligence and desire for a positive tone here. I feel, however, that you bite the hand that feeds you. If the holographers posting here were not "helpful to one another", they would simply join the vast majority of other holographers who just ignore forums like this. Where would you be then?

You make good points regarding the hanging table idea. Their inclusion with your original posting of the idea would have satisfied my request. I remember, in fact, people hanging their turntables from the ceiling on springs (remember vinyl records?). Not very practical.

There was a bunch of technical discussion regarding the stiffness calculation for an aluminum can table at the beginning of this thread. Perhaps someone can comment now on the requirements of the cables needed to effectively support and isolate the weight of a typical holography table (made of steel skins and aluminum cans, possibly filled with sand or something). What happens when you bump the side?

[JohnFP]

Actually Bob, although I have not had time to get into the lab lately, I will say that I am actually going to try the hanging table. I hope to contruct two idential tables. One on the ground and one hanging. The former an inner tube on the ground with a 18" square stepping stone, the latter a hanging table, inner tube then the 18" stepping stone. I have two 5 mw HeNe's. I will build and interferometer on each and place them side by side. I hope to monitor the fringes after certain induced vibrations. I will try to record my findings on video and let you know. This will test the additonal dampening of the hanging technique. I wish I had John's table as I would like to try a rigid table without the inner tube compared to the inner tubes for testing tube against hanging. Maybe I can borrow a breadboard from the lab up the street for a couple of days and see if John's table could benefit from it. It could hang from a tubular pyramid and be modular and portable.

When I was doing photo resist H2's with exposures of 50 minutes and greater, there was no far fetched idea that was not mulled over. I have always wanted to try the hanging table and now I am fired up to do it. I actually have to thank you for the kick in the butt.

Health, Peace and Great Prosperity to you!!!

[BobH]

That's the spirit!!!! I'd love to hear the results.

[Bruce]

I have an idea! Lets start a holography forum where people can talk about the holograms that they are doing!

B.