# Characterise Magnetic Field Outside Electromagnet core

1. Dec 2, 2011

### Pdub

Hi all.

I am shortly to begin work on my engineering FYP in automatic control and I'm hoping for some advice to point me in the right direction. The project will initially be an electromagnetic suspension rig (cylindrical winding with soft core suspending a permanent magnet below it via feedback control). I then want to try to acheive off-axis suspension by using 2 or more windings side by side.

What (I think) I need to do to model the system is

1. Describe the B(?) field due to the coils accurately both on and off the axis
2. calculate the total force on a magnet of given strength and orientation placed in the field by integrating over it's volume.
3. repeat the above 2 steps for the permanent magnet's field acting on the electromagnet's core.

I'm looking for the best approach to take. I've found the Biot-Savart law but I'm unsure how it deals with the effect of the core. I've also found equations for B due to a magnetic dipole here but I think modelling the electromagnet as an ideal dipole will be an oversimplification as the air gap is going to be smaller than the size of the electromagnet?

Any pointers would be greatly appreciated. I know I've got a fair bit of learning to do which is fine, it just helps to know I'm going in the right direction.

Pdub

Last edited: Dec 2, 2011
2. Dec 2, 2011

### rude man

Your best bet is to get some good simulation software.

I wonder why you picked a permanent magnet for your suspended mass. You could have picked another soft-iron mass instead, which woud at least have avoided questions regarding orientation control of the suspended mass? The force on the iron mass would be pretty close to proportional to the applied current, yielding a more or less linear relationship between current and force. Makes the control system design a lot easier ... but then maybe you were told to use a p.m.

Generally, calculating B fields is difficult uless the flux path is contained. That's why problems dealing with the B field inside a solenoid, for example, specify an "infinite"-length solenoid. Basically, magnetic circuits are far more difficult to compute than electric ones, simply because containment of the flux is usually sloppy compared to electric currents.

I don't know enough to comment on the off-axis alignment problem.

3. Dec 2, 2011

### Pdub

Thanks for your response rude man

I picked a pm for the suspended mass because I thought it might allow larger air gap but I've got the option of changing to a ferrous mass if I have to. I guess I'm looking at the extra non-linearity as a learning opportunity rather than a PITA at the moment (it's still really early on!)

"Difficult" is what I keep finding as the answer to my question regarding the B field wherever I look..textbooks, wikipedia all agree.

When I first started reading into it I thought "sure, FEA is the way they do it in sim software but surely if I'm keen enough I can do it by classical means". I suppose now that I think about some of the mechanical applications I've seen for FEA it's starting to make sense that it might be the only practical method.

If anyone has any other thoughts I'd still like to hear them though. I've got about 12 months to figure it out....