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Athens
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Hello there people,
I am an architecture student from Greece, currently on my thesis project, where, I want to create a kinetic spacegrid/canopy. You can see CG pictures of it here http://at-nowhere.tumblr.com/post/22669358307/kinetic-space-grid 
and an hd video here http://www.youtube.com/watch?v=zdqsBtYTC94&feature=youtu.be&hd=1

My intention is to make this interactive using something like kinect, to read people inside it, and produce reactions by the grid.

I want to do something like that, a grid the nodes of which can move in XYZ directions in space, and I have to make working a model of it.
For a hypothetical 1:1 realization I was thinking of hydraulic linear actuators. For my project, I have to build a model, probably around 1:20 scale, so I am thinking of linear actuators, or "home made actuators" build to resemble the large hydraulic ones. ( a double cylinder, with a spring inside it, and a threat connecting the spring to a stepper motor to control the force compressing it.)
 
I don't need this thing to be able to bear huge loads. I am in some distress, because I don't know from where to start, and what to buy.
Given that I would need at least a dozen, maybe a couple dozens, the cost of buying these, actuators or steppers goes up high.

So my question is,
-what kind of machinery would you propose for this project (steppers, actuators or something else)
-if you know where I can find cheap ones (actuators retail for something like 100$ each, not including the driver) - given also that I am in Europe,  (of course I am interested in something that can be controlled by arduino)
-if you know of motor drivers that can control as much of the above as possible (I couldn't find motor drivers that control more than 3 components, so this wat I end up with 4-8 expensive drivers to do the job, which I think is not very practical)

thats all,
thanks in advance
constantinos
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Hi Constantinos-

I read about these linear actuators recently- from what their site says they operate like linear stepper motors, or there are versions with limit switches or even with no feedback at all. Maybe the site will give you some ideas?

http://www.firgelli.com/products.php?id=41
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For my project, I have to build a model, probably around 1:20 scale, so I am thinking of linear actuators, or "home made actuators" build to resemble the large hydraulic ones. ( a double cylinder, with a spring inside it, and a threat connecting the spring to a stepper motor to control the force compressing it.)

You'd probably be better off using threaded rod and a captured nut; see here for a small example, which can be scaled up:

http://blog.davidjbarnes.com/2009/03/making-linear-actuator-for-robots.html

...also, do a little bit more googling and research on your own; many people have made similar actuators (also, something similar to them have been used for low-cost homemade CNC machines to move the X/Y/Z axes). Such homemade systems won't stand up to heavy usage (such as in a full-sized implementation of your system), but for smaller loads that don't require a lot of accuracy, they work fairly well.

If you need to track position (and/or end-stops), that will add complexity and cost, of course. The driving motor can be nearly anything, of course. You can do direct-drive if you have a powerful enough motor, but most designs (including commercial ones) use belt, chain, or gear drives, indirectly off the motor with the motor offset, or planetary gear drives with the motor in-line with the shaft. Also, commercial linear actuators do not use regular "all-thread" cheap threaded rod (like you would find in a hardware store or the like); they instead use ACME threaded rod, or on the more expensive actuators, "ball-threaded" rod (my terminology may be off on that last one; I can't remember what it is technically called, but the "traveling nut" uses ball bearings to reduce friction, and both of those have limited lash-back).

Finally, to also help your searching: Hydraulic (and pneumatic) cylinders, while technically being "linear actuators", aren't typically referred to as "linear actuators"; what is typically meant by "linear actuator" in most cases means "electrically actuated", typically via a motor of some sort (and generally a motor that rotates - if you dig deep enough, you'll run across linear actuator "solutions" that use linear motors, which act/work kinda like a stepper motor that has been "unrolled"; these kinds of actuators will not work for your application - besides which, they rarely are inexpensive).
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Athens
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thank you very much for your responses,
i took some time to answer to consider alternatives.

I did consider frigelli actuators, but given that I'll need more than 10 elements, it'll get pretty expensive.

cr0sh, thanks for your link,
it seems like the cheapest solution from all although it has to be done with a stepper motor instead of a regular DC for better accuracy.
do you know how the call these PVCs?

I have ordered 3 stepper motors and their driver, to begin some testing.

a different solution is a thing called bio-metal. its a spring or threat from an alloy that extends when heated, or when you pass electricity through it. (though it can not be used as a load bearing element, just like a mini muscle)
I'll probably check that too, its the simplest and probably the cheapest method (it doesn't need a driver, just plug and play), although I dont know how accurately one can control its deformations.
the site of the company is this :
http://www.toki.co.jp/biometal/english/contents.php
its similar to shape memory alloys.

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