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Author Topic: Build your own solenoid / linear voice coil actuator?  (Read 8898 times)
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Manchester, New Hampshire
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Hey guys,

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?
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Being as its a replica, maybe you could fake it with a servo & control arm inside of a tube so it looks like that. The up & down length is then controlld by the radious of control horn used. Mount the servo so the motor is perpindicular to the arm travel. As the motor turns left & right a control attached to a control horn on the motor would move lengthwise in the tube. Position can be controller with servo library to make it smooth & coordinated with the other tubes.

Winding your own magnet is possible, coming up with a control circuit for current flow to keep the moving part from just going extreme to extreme is the tricky part.
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How physically big does this need to be??

I agree with Crossroads that servos are probably the best solution.

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Around 6" in diameter, and 6" tall.  I figure the tubes would need to be around 1/2".  I don't think even a sub micro servo would be able to fit in a tube that small.  But perhaps I could fake it by hiding the servos behind the bottom plate and having the pushrods come up through it and through the tubes.

This is what I'd like to build:


Around 47 seconds in he freaks out, and you can see the linear actuators inside the model attached to his eye.  I want to model them in the prop because you can see them through large holes in the sides of him.  Wheatley is around 12" in diameter, so that puts the eye around 6".
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What about a miniature motor, attached to a small threaded rod, and a traveling nut? The following link will give you an idea of how to do it; there are other examples as well on the internet:

http://www.instructables.com/id/Quick-and-easy-electric-cylinder-prototype-.../

You might also be able to find an actuator small enough from these guys (if you're willing to spend the money - LA's aren't cheap):

http://www.firgelliauto.com/

They might also give you some ideas on how to homebrew your own as well...

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Attaching a motor to a rod that moves a nut is the same kind of setup that most linear actuators use, and it's really slow.  There's also no easy way to know when to stop it, and it would be prone to mechanical failure with repeated use.

And the actuators from Frigelli are much too large, and expensive.  Even the 'miniature' ones are too big.  And I don't want to spend over $1000 on this.
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Hm...

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My understanding is that voice coil motors are essentially constant force devices at low speeds, so you would need sensors to tell you the position of each motor, or 3 sensors to tell you the positions of each of the three "corners" of the platform.
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Quote
perhaps I could fake it by hiding the servos behind the bottom plate and having the pushrods come up through it and through the tubes.

Yeah, that's what I was visualizing..
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Is there a pneumatic-valve shield? Those legs beg for hydraulic or pneumatic power. The Arduino would only have to coordinate the valves, the real power would flow from a compressor and may be 150+ psi without big motors except for an external compressor or power totally from compressed air or nitrogen tank, for a while at least.

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Quote
perhaps I could fake it by hiding the servos behind the bottom plate and having the pushrods come up through it and through the tubes.

Yeah, that's what I was visualizing..

I'm thinking the same.
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3/4 thin wall PVC will admit 5/8 plunger. Wind 3 layers on the pvc and use 5/8 neodymium magnets. You now have a voice coil actuator.
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Check out how voice coils in hard drives work. They do very precise positioning through use of the Faraday/Lenz Law.

 

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"Reply #10 on: December 20, 2011"

You guys are a little behind on the suggestions smiley-cool

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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
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Yeah, but I'm a little behind on constructing Wheatley so it's all good. smiley
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