I'm interested in building one of these:http://en.wikipedia.org/wiki/Stewart_platform
And I want it to look like that too, with the round tubes, because it's for a replica prop where they'll be visible. The height of the thing would be around 6" so these have to be small too.
I've been researching for a while and I haven't really been able to come up with a good way of doing it.
Standard linear actuators which use a ball screw are big, expensive, and slow.
Solenoids are also expensive and I can't seem to find ones which extend a reasonable distance.
Pneumatics are no good because I don't want to deal with compressed air and they'd be noisy.
I might be able to roll my own with a servo... fit a pipe inside a pipe and put a pushrod inside to move it, but it's not a great solution.
I just found out about these things though - The linear voice coil motor:http://www.pwr-con.com/voice-coil-motor.htm
Unfortunately the ones I've found so far are really expensive. But they look really simple, like a solenoid.
I'm wondering if there isn't some way I could construct something like this with cheap materials.
I know folks use coils of wire for a lot of things in electronics. I've seen long ones used in radios, and there are probably ones people use for electromagnets. And if you want to go really small, there's inductors which I assume work pretty much the same? I presume there's also tiny coils in speakers, which is where I presume the term "voice coil" comes from.
I'm also aware of there being rare earth magnets which are round and have a hole in the center. I could see some kind of setup where you have one of those on the end of your moving shaft, possibly with a rod through it to help guide it, and stop it. The electromagnet, possibly a small speaker coil, could be in the base.
I'm basically looking for something stupidly cheap and simple to put together, but I'm not sure about some details like what direction the poles go in on something like a speaker coil and how strong a force that could exert on the magnet. I would assume if it does work though that one could use PWM to adjust the speed and force, though one might not be able to actually determine position. For my purpouse, being able to extend fully or retract fully would be good enough. I could get a bunch of different angles by choosing which cylinders to extend and retract, and that's all I want.
So, any thoughts? Possible to jerry-rig something like this on the cheap? Or maybe you know of a source of cheap solenoids that would do the job?