Electromagnetic linear actuated beds?

Just thinking with the fingers here - Would it be feasible to hand-craft electromagnetically linear actuated beds for the X and Z planes?

I'm visualizing a batch of rare-earth magnets glued to the underside of the platform and some hand-wound electromagnets mounted to the bottom frames for each axis.

I'd think with some sort of high-resolution optical positioning system (cannibalized from a printer, perhaps?), you could compensate for relatively sloppy positioning, assembly tolerances and inertia via software.

Unless I'm missing something the parts cost on something like that would be fairly low (quick search shows 50ct of rare earth magnets for $10 on Amazon). The big unknown would be the position sensors.

Could you even leverage the existing stepper motor boards? I'm guessing you'd need more than 4 outputs/electromagnets to really position things correctly.

It would have the benefit of being pretty robust and cheaper to repair than steppers/screws/cables/etc.

Not having any experience with electromagnetic linear actuators (beyond pinball bumpers), I've no clue if it'd actually work or what the downsides would be.

Neat idea. Steppers have a fair bit of mechanical advantage with a small pulley. They get many coils / mm that way; I think a 200 step stepper has about 0.2 mm/step res when full stepped (I half step mine) with the usual pulleys; that would be about 0.8 mm per coil on a linear rig, I think. If we can get away with 16x microstepping, that might be as much as 6.4 mm per coil; that might be do-able. I need to actually draw this out first; I'm just guessing here.

Normally, this sounds like a lot of extra hardware and work, but if we can print circuits a la Spoolhead, it might actually make sense.

Couldn't you use PWM/Analog voltage control to the electromagnets to control the position of the bed magnets exactly at any point between pairs? It'd be an active feedback loop, constantly adjusting the "power" of all of the electromagnets to slide the bed in one direction or the other. The magnets/electromagnets wouldn't have a 1:1 alignment.

Sort of like this: Google Docs Drawing

... read the thread about "Reprappable linear drives" - especially the atached images in the 1th and 15th posts ...

Some years ago i found a linear XY-table with a cross-hatch-milled iron-table of nearly 1x1m size and two planar linear motors similar to the image in the 1st post arranged in a moving head ... and two sensors scavanged out from optical mices for positional feedback ...

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Viktor
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Call for the project "garbage-free seas" - [reprap.org]

VDX Wrote:
-------------------------------------------------------
> ... read the thread about "Reprappable linear
> drives" - especially the atached images in the 1th
> and 15th posts ...
>
> Some years ago i found a linear XY-table with a
> cross-hatch-milled iron-table of nearly 1x1m size
> and two planar linear motors similar to the image
> in the 1st post arranged in a moving head ... and
> two sensors scavanged out from optical mices for
> positional feedback ...


Ah! I did some unsuccessful cursory searching ("electromagnetic" wasn't anywhere in that thread) before posting - but that's exactly what I'm talking about. Sounds like the thread died without any real "here's why it will/won't work" conclusions. Once I'm done with the McWire, I'll dig around and see what I can find. I'm a far better tinkerer than programmer, so I'm pretty sure that's where I'll get stalled out.

I think Zach blogged somewhere about a new sensor that used a simple strip magnet for position sensing, maybe a combination of the two would be a good approach.

Hey,

Yeah, a SpoolHead-esque device would be way better at printing linear drives like this than stepper motors. But there's some drawbacks.

- Linear motion doesn't let you use gears or screws for a mechanical advantage. So to get a strong enough force, you need a lot of current, which can cause heating problems. It would probably be quite slow to start/stop.
- The motor would be large and heavy compared to an equivalent stepper motor, probably.
- You'd almost definitely want to use iron to conduct the magnetic flux and boost the force. But that's not printable for now (unless we have a way of mixing iron powder into the printed filament). Of course you could use scavenged iron plates, if you're building it by hand...
- Even using feedback, it would probably be quite difficult to control a surface where the magnets aren't placed with any precision. The force varies a lot with distance away from the magnet, so even a closed-loop system might struggle to keep its position.

Google is your friend.
"Sawyer motor" brings up many relevant hits. These are certainly doable, but making the platens will take some effort.