Idea: Floating Build Platform

I wonder...

Is a floating build platform practical?

(Warning: this idea is slightly heretical, as it mixes water and electricity. Furthermore, I may be completely wrong.)

Here's how I think it might work:
Start with a shallow basin and a buoyant platform half as large in the plane. Both should be non-porous and poor electrical conductors. Affix a set of parallel unshielded wires (PCB?), wired in parallel in the basin. Affix another set of parallel wires on the underside of the table. Fill the basin with an adequate amount of water, perhaps with some salt added to aid conductivity. Apply small amounts of electricity to both sets of wire.

I suspect there is a method by which the current can control the position of the table. The table must be buoyant enough to not be submerged by the electromagnetic forces used to position the table. The force generated must be sufficient to hold the table stable and move it against the extruder's output stream. This is a modest force. Of course the table must be stopped and started precisely, but precise control of electricity is certainly available.

(The apparatus described above may only allow a single axis of control. I would like to get opinions before attempting to describe the second axis of control. I am sure if this method is possible that it has already been developed, probably a century ago. I wonder whether the wire needs to be coiled in some way to be adequately effective.)


Pros
Fewer parts than mechanical planar movement, notably fewer steppers
Auto-leveling
Speed?
Price?

Cons
Electricity and water
More custom parts (maybe.)
Heated build platform --> hot water
Conducting pieces affected by decay by 'electroplating'
Scalability may be somewhat limited

Thanks
--Kris

WIll current be flowing through the water? I have done a bit of dabbling with simple electrolysis, but have never seen noticeable forces between the wires immersed in the electrolyte. Also, descriptions of electrolysis tanks do not mention having to compensate for such things.

So, would it be magnetic forces between the wires that control the position of the floating part? That might work, but you would need more than two wires.

This sounds like the kind of idea that an afternoon and a few dollars could prove or disprove. It would be a lot of fun to do, too.

fdavies

As I read it two ideas seem to be mixed, one of magnetic forces which would suggest a linear motor to me (search for 'planar motor' to find 2d linear motors) the other seems to have to do with forces created by electrical current through the solution. See here for some related experiments (do note the 500 - 7000 amps of current used): [www.df.lth.se].

Also I would expect a lot of trouble with suppressing waves in the basin...

I am thinking of the way needles will float in the direction of the earth's magnetic field. As well the propulsion of mag-lev trains. Whereas the trains are propelled by massive banks of electromagnets used to propel their tons of mass, here the forces are miniscule, only needing to overcome the inertia of a platform floating in water.

More than one wire would be needed, at minimum two on the platform and 3 in the basin. Probably significantly more. At maximum, enough wires to cover one-quarter (?) the exposed surface. I believe it is important that the build platform wires and basin wires be in series so that the frequency of the current is identical.

The exposed wires would be separate by a minimum distance, roughly estimated at 5mm. They would be in direct contact with the water. I have no plans to manipulate the z-axis via flotation.

Experimentation is one course of action. Another is to pull down the physics book from the shelf. I'm not certain how to generate the proper current without a power supply. I had thought that this had been tried before and that someone could point me toward a reference. However experimentation would probably be quick and easy.

Time to go find some batteries and strip some wire.

--Kris

>> here the forces are miniscule, only needing to overcome the inertia of a platform floating in water.

If you want any kind of reasonable print speed, you're going to need much greater force than you imply here. One does not require much force if you simply want to move a thin wire (a needle, a 1/4 gram or so), a few millimeters, over a few minutes. It requires several orders of magnitude greater force to move a platform capable for reasonable RepRap prints (100+ grams), many centimeters (10-20cm) in a few seconds. I think you may be underestimating the force needed.

My experiment did show that the force was a bit inadequate, but it was not completely ineffective. The platform moved about 5 cm and rotated 135 degrees in about 15 seconds. Not bad for a 9V battery and some takeout containers with wire running through them, but still not great.

The resistance of the (super-saturated) salt water is much higher than I thought, and a bath of mercury is just not practical.

The battery became hot to the touch and was beginning to soften, which indicates a need for quantitative analysis

I was able to get the float to rotate a few times, always aligning the float wires 45 degrees to the basin wires. The float drifted to one end of the basin away from the rest position imposed by the wires supplying power to the float. I am not certain that the lateral movement was not caused by the rotation.

Reversing the current did not change the outcome, so a switch would need to be added to flip the direction of the current through the platform relative to the base.

I'll start collecting parts for a more elaborate rig. I've run out of my consumables: salt and batteries. (No way I'm going to hook this up to the wall just yet.) Maybe investigate resistances of common liquids. Build some better apparatus for the next test. Investigate a variety of wiring variations.

What I'm still wondering is whether the force can effectively brake the movement of the platform.

--Kris

What happens when one prints a large, capital-letter "T" shape, taking up as much of the bed as possible? Wouldn't the boat start to tilt with the off-center weight?

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-Sebastien, RepRap.org library gnome.

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Remember, you're all RepRap developers (once you've joined the super-secret developer mailing list), and the wiki, RepRap.org, [reprap.org] is for everyone and everything!

The degree of tilt would depend on the weight of the model relative to the platform. Of course the platform will always have a larger footprint than the model. In today's experiment I recessed the wire path flush with the platform to try to avoid shorts. Of course friction between the platform and basin would still be a significant problem if it were to occur.

It's clear the floating platform idea is not a good general purpose solution, but I hope it may be adapted to a mini-size fabber. This would fit with removing several motors from the standard design.

If the degrees of control are limited it might be adapted to a polar system.

If does not work for a fabber it still might turn into a really neat naval warfare chess set. Are there chess sets that move their pieces through magnetic means?

--Kris

Are there chess sets that move their pieces through magnetic means?
Yup. [www.amazon.com]

It's clear the floating platform idea is not a good general purpose solution, but I hope it may be adapted to a mini-size fabber. This would fit with removing several motors from the standard design.
Well, you have my support. Not because you've convinced me that it is practical, but because whimsical projects are welcome here. I hope that is ok.

Good luck!

Also, you may have more luck trying to figure out how to print pneumatic or hydraulic stepper motors and valves.

---

-Sebastien, RepRap.org library gnome.

---

Remember, you're all RepRap developers (once you've joined the super-secret developer mailing list), and the wiki, RepRap.org, [reprap.org] is for everyone and everything!

... you can build a complete stick'n'slipp-free moving plattform by glueing some magnets at the bottom and add ferrofluid on the magnets, so they glide on the fluid cushions.

It's only for XY translation, but with aditional moving magnets under the base (or a switching coil-array) you can move and tilt the plattform across the working area ...

My recommendation is to use magnets at both ends of your axis and ferrite core electromagnets attached to your basin. You would need a feedback system to read position, but you could use this setup to pull the platform in the desired direction and the opposite magnet to stop it.

I have seen a system similar to this used to make a globe float in mid air. In the globe was a magnet running from poll to poll. Above the globe was a electromagnet that would lift the globe. Below the globe was a sensor that would detect the position of the globe. Gravity was used as the opposing force. If the sensor detected that the globe was getting to high (too close to the magnet) it would turn off/down the magnet. If the sensor detected that the globe was getting to low (too close to the sensor) it would turn on/up the magnet. This was happen hundreds of time per second creating a stable position in the air. But if you touch it, it would bound and wobble until it restabilized.

I hope this works out for you. have fun.