Choosing an electromagnet

[Rachie5272]

I need an electromagnet for a project.  It needs to be able to hold a ~2 kg block of steel securely in place, and then drop it.  Around 20 N of force.  It needs to do this through a ~3 mm thick sheet of plastic, and be powered by a few alkaline cells.

I bought a couple little electromagnets which are quite powerful for their size, but their field strength drops tremendously past 1 mm or so, and are useless for this application.  I'm guessing this is because they are based on a magnetic circuit.

I thought about using a couple permanent magnets to assist the electromagnet, but this is a failsafe mechanism, and I need to be absolutely certain the weight will drop when the battery runs down.

Can anyone help me design this?  I have no idea where to source good electromagnets.  Neither Mouser nor Digikey seems to carry them.

[madshaman]

Hi, here's an idea:

Get a larger cylindrical neodymium magnet and some large guage magnet wire (mouser carries the magnet wire, google for the magnet), wrap the magnet like you would a regular electro magnet, then to turn it [the permanent magnet] "off", dump a lot of current into the coil so as to create a field in opposition to it, it will probably be enough to reduce the strength of the magnet to drop your steel plate.

Calculations of how much current you can safely dump into your coil without destroying the neodymium magnet by overheating it or exceeding the limits of the magnet wire's coating is up to you and google.

[SeanB]

Not doable with the 3mm gap and having to run from alkaline cells, you will have to think in terms of having a permanent magnet just not powerful enough to do the job and then add the electromagnet to add enough extra force to hold securely. Try looking for fail secure Maglocks, then strip them and power the coil in reverse so that instead of negating to the integral magnet used to make it fail secure you are adding to the force. That will them likely be able to operate through a 3mm gap of non magnetic material.

Otherwise a standard fail safe maglock runs off 12V and uses around 1A, and then you will have to make a set of pole pieces to match the E core layout and have the ferrite or neodymium magnets in the gaps of the core to provide the extra magnetic field.

Magnets are as close as eBay for neodynium magnets, or Adafruit sell supermagnets, along with most suppliers. They probably even have the maglocks, though you will probably find them either onlin or at a security suppliers.

[smashedProton]

I think that it would be better to have a solenoid control a latch that would drop the plate.  It is too much to ask of alkaline batteries to get 20 N of force.


Edit:  Maybe you could get an excuse to use explosive bolts!

[Rachie5272]

Activating an electromagnet to drop the weight isn't suitable for this application.  I need a passive failsafe, meaning even if all the electronics fail, it will still drop the weight when the batteries run out.

The electromagnets I have get 30 N from 6 volts at 200 mA, so I know alkaline batteries are capable of supplying the force.  It's just a matter of getting an electromagnet which can work through 3 mm plastic.

I'm making a submarine, and the plastic is the hull of the vessel.  I'm trying to avoid any sort of hull penetration whatsoever.  This means I don't have to deal with weakening the structure, or trying to waterproof the electronics.  So, running wires through the plastic is out.

Although I would love an excuse to try explosive bolts, they violate both the passive failsafe and hull penetration rules.

[smashedProton]

You could use active electronics to set a latch, then have a spring kick the latch if the magnet dies.  That way the magnet wouldn't have to take the whole 20n

[IanB]

The electromagnets I have get 30 N from 6 volts at 200 mA, so I know alkaline batteries are capable of supplying the force.  It's just a matter of getting an electromagnet which can work through 3 mm plastic.

That's not "just" a matter, that's a big deal. Magnetic force falls off very rapidly with increasing distance. To overcome this you would need a magnet that has a large core cross-section relative to the 3 mm distance.

A magnet with 30 N attractive force in direct contact will not generate anything like 30 N when there is a gap between the magnet and the test piece.

[Rachie5272]

Yes, I know magnetic fields drop off with the square of the distance, but this particular magnet seems to weaken even faster.  Probably because it's meant for a magnetic circuit, which requires near-direct contact.  I get practically nothing with 3 mm distance.

So, any idea what I can do?  20 N is just an estimate.

[Andy Watson]

Yes, I know magnetic fields drop off with the square of the distance, but this particular magnet seems to weaken even faster.  Probably because it's meant for a magnetic circuit, which requires near-direct contact.  I get practically nothing with 3 mm distance.

Remember there are two poles to the electromagnet, when you open up a gap of 3mm that's 3mm in each pole - a total of 6mm, and as has been pointed out above, an air gap will dominate the magnetic circuit.

So, any idea what I can do?  20 N is just an estimate.

Introduce some form of mechanical advantage. What about straps that wrap around the hull - it would take far less force to retain the straps than it would to hold 2kg.

[amyk]

Introduce some form of mechanical advantage. What about straps that wrap around the hull - it would take far less force to retain the straps than it would to hold 2kg.

Or some sort of latching lever mechanism - something like that on a mousetrap might be ideal.

[Rachie5272]

But how can I actuate the mechanism through the hull?  The electromagnet won't even pick up a paperclip through 3 mm of plastic.

[SeanB]

Can you have a plastic intrusion into the hull so that a steel pin goes in, and the electromagnet is wound around the interior protrusion? That way you can have a pin mounted ballast mass, and release it when needed.

[IanB]

The electromagnet needs to have a closed magnetic circuit between the magnet poles and the thing being held, and there needs to be a large magnetic "current" in that circuit.

A horseshoe magnet is the typical way of getting a closed circuit. Both N and S poles of the magnet are attached to the target so that the field lines go in a circuit from N to S and back to N. Compact magnets can be like a small cylindrical "horseshoe".

Getting the largest magnetic "current" requires more turns on the magnet, more current in the turns, and a larger core cross-section. The air/plastic gap will have less impact of the size of the gap is small compared to the size of the magnet. Your magnet may need to be several centimeters wide, for example.