Build your own solenoid / linear voice coil actuator?

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.

How physically big does this need to be??

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

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".

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:

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):

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

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.

Hm...

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.

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..

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.

terryking228:

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.

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.

Check out how voice coils in hard drives work. They do very precise positioning through use of the Faraday/Lenz Law.

"Reply #10 on: December 20, 2011"

You guys are a little behind on the suggestions 8)

Yeah, but I'm a little behind on constructing Wheatley so it's all good. :slight_smile:

A 5tpi ballscrew could be pretty fast. Not cheap, though.

Building a good Steward platform is actually a very expensive affair, a good one would require 12 spherical roller bearings:
http://www.hephaist.co.jp/e/pro/ball.html
Or you could try and build a few of your own, but I would strongly discourage that.

For a Stewart platform you are looking to move in six degrees of freedom accurately to any location, so you would need accurate position control for the actuators.

Voice-coils - again, like pneumatic pistons are a force controlled devices. You only have control over the current, which in turn controls the force, you'd have to put in two sets of sensors for position control - a secondary pick-up for velocity and a linear position sensing device (LVDT / optical etc) to do it properly with feed back control through the arduino, you'd also need to have reasonable expertise in control theory to do it right. Also, you must remember that the faster the coil is moving the more back-emf it is generating - i.e. the coil is ITSELF generating power because it is moving. The back emf is dependent on the velocity of motion, and you'd have to account for it in your circuit as well as in you control algorithm. Although only a single optical linear pick-up should be OK, but you would not have sufficient "gain and phase margins" - control jargon for "you wont be able to respond quickly enough to changes". And NO, a "simple PID controller" would NOT work.

With regards to your second question of building a voice-coil actuator yourself: Hmmm ... easier said than done, some commenters have suggested a PVC pipe a bunch of Neodymium magnets - if it really was that simple people would not be selling them for 100s of $, and there wouldn't be people willing to pay those 100s of $ for them!

First, Newton's third law: For every action there is an equal and opposite reaction - i.e. for every action force, there must be an equal and opposite reaction force. For the force that the voice-coil would be exerting, there would be an equal and opposite reaction force. This force is actually acting through the coils, this is where Lorentz force comes in to place. The reaction force experienced by the coils is actual radially OUTWARD.

The trick to making voice coils is getting the winding of the said coils right by balancing its current carrying capacity with the winding tension, the heat carrying capacity of the substrate (no PVC would NOT work) on which the coil is wound with the mechanical and electrical properties of the coil and its geometry. So, yah! Muchos experience in doing helps.

There is also the issue of mechanical integration - there is no real bearing element between the outer sleeve and the inner moving sleeve - if you don't put in some kind of sleeve the coils keep rubbing, get worn out and short (been there don it). There is also the extraordinary risk that some magnetic element might "fall in" between the tiny gap that exists. Now your voice coil is USELESS. You'd NEVER be able to use it again since you cannot get the bloody screw / iron filing / machining shaving out (been there don it!). Now your 100s of $s voice coil is a paper weight and quite an ugly one at that!

Bottom-line: you are better off using a stepper motor to move a screw rod, you only need to know "home" and then keep track of the number of pulses you send out for position - no feedback control - only feed forward. Of course if the loads are too much for the motor to move, then you would end up with "missed pulses", i.e. you'd send out a pulse but the motor would not have moved. You'd keep having to move back to "home"/zero position to regain your reference, OR put in an optical linear pick-up and put in a very basic feedback control, even a PID would work.

Mechanical integration is not easy with stepper motors either - what are you going to use to prevent the body of the motor from itself rotating if you are using spherical joints? You'd need to "anchor" the body of the motor with linear bearings to the linearly moving element.

BTW: If it is not to much of a secret, what is your application for the Stewart platform?

CrossRoads:
"Reply #10 on: December 20, 2011"

You guys are a little behind on the suggestions 8)

It takes a while to work though the entire forum history and reply to every post...!

Hey, if someone posts on a 5 year old thread, but it is a good answer, I'm all in.

AmitG:
BTW: If it is not to much of a secret, what is your application for the Stewart platform?

It was on page 1 of the thread:
http://forum.arduino.cc/index.php?topic=83606.msg626707#msg626707

I never did get to building it. I'd still like to eventually though.