Electromagnet sliding

[robopm]

Hi all,
I'm trying to make a buggy that adheres to a steel wall using an electromagnet. The problem is that the buggy needs to move along some tracks. With an air gap, we're not getting enough force, and without one, the friction is too much and the buggy can't move. Anyone know a way to get around this? I was considering making special shoes with rollers on them, but I'm not sure if I won't just have the same problem again.
There's an air gap of ~1.5mm with the current setup, and the electromagnet is a bi-polar 4x1" magnet, if this helps for some project scope. Thanks!

 

[MagMike]

A picture will really help.

What is the load capacity rating on the electromagnet? Which way is is positioned? Any steel in the buggy that will absorb/shunt the magnetic field from the electromagnet?

Reducing the air gap will help. Any reduction in the air gap will help.

 

[robopm]

The electromagnet is off the shelf. It's a magnetechcorp PL-104013-24, rated for 200lbs, with a simple plain carbon steel shoe on it. The magnet will be positioned on a wall, however the wall's angle will depend on the specific job, as this is intended to be an all purpose piece. The buggy itself is made of aluminum. I might be able to get the air gap down to 1mm, but after that, uncertainties mean that I may scrape the wall with the shoes, and I don't want any sticking. Is there any lubricant you know that does the job well?
Because I can't post everything here without a NDA I can't provide a proper sketch.

 

[IRstuff]

Note that a magnet really has no preference to where it sits on a steel plate, so the only thing that's really holding it in place is the friction. You might get something from eddy current resistance, but, again, probably need way stronger magnet for that, or possibly a thin copper film over the steel. What kind of steel is it?

Once the friction is removed, you essentially require infinite field strength to keep something from sliding, so you have to depend on the friction between track and wheels.

TTFN
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[israelkk]

If I am not wrong the 200lb is for ZERO air gap. If you introduce an air gap it will decrease by many folds. Try to put a non magnetic (paper, card board, plastic, aluminum, etc.) 1.5mm thick sheet between the electromagnet holding face and a ferromagnetic (iron/steel) thick plate and check the separation force needed to separate the plate from the electromagnet. This is the hold force of the magnet at 1.5mm gap.

 

[robopm]

I have tested the air gap using cardboard already, which is how I know approximately the force I have to deal with now.
IRstuff, do you have any suggestions for how to reduce friction between the magnet and the steel wall? If i can make that negligible, I would be able to achieve a near 0 gap.

 

[IRstuff]

" If i can make that negligible, I would be able to achieve a near 0 gap."

But, again, it's the friction that stops the sliding, not the magnet. The magnet's sole purpose in a kitchen magnet is to create the friction that keeps the scraps of paper from sliding. In fact, a smooth finish on the refrigerator makes things much worse. The bottom line is that you have to have mu*m*g to keep the buggy from sliding, therefore, you need to have your buggy's wheels to keep that from happening. You could possibly coat the magnet with PTFE or Delrin, but your wheels then have to have sufficient friction alone.

TTFN
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[robopm]

I should perhaps clarify, many of the walls which this buggy will be climbing will leave the buggy hanging inverted. The purpose of the magnet is to connect the buggy to the wall, but it is the buggy's wheels which will create friction and motion. The magnet is there only to counteract gravity, not to render the buggy immobile.

 

[MagMike]

Try to find another electromagnet with the poles farther apart. The one you identified is ideal for zero to near-zero air gap.

What material are the steel walls made of?

Can you magnetize the wheels of the buggy?

 

[robopm]

We'll have a number of steel walls, but they will mostly be plain carbon steel, and about a quarter inch to a half thick. Also, we are using two electromagnets placed side by side (SN-NS configuration). The buggy has some magnetic wheels, but they are not particularly strong, and too strong means that the buggy is hard to place and remove after use.

 

[MagBen]

Magnetically, this is a pretty straight forward question, just as MagMike pointed out, you would need to decrease air gap (including aluminum and any other "non-magnetic materials") or, order a stronger magnet to obtain a stronger holding force. I believe the plain carbon steel is part of your magnet, right? If your buggy is made up of Al (or other non-magnetic materials), you donot need to worry about shunt or deture of the magnetic flux pattern.

 

[robopm]

That's good to hear atleast. I know that a bigger/more powerful magnet will aid, as will reducing gap. Unfortunately sizing constraints and tolerances mean that there must be some amount of air gap. That said, one idea I have had is to have rollers beneath the magnet to contact and conduct. Has anyone any experience with this, and how it should be implemented?

 

[Compositepro]

Some options you might want to consider:

1. Use vibration rather than wheels to move the magnets in the same way that a vibratory conveyor will move material up-hill. The magnet would essentially make dozens of hops every second without ever actually leaving contact with the surface. The line of action of the vibrator would be at a 45 degree angle to the plane of the surface. Two vibrators allow for steering.

2. Use a walking action where each pole face is made of two bars. One bar retracts and moves forward while the other bar is still in contact with the surface.