A Magnetic Levitation Toy

Here's an idea. I'd like to see if anybody has thought of it or if I'm missing something.

A ferromagnetic substance concentrates magnetic flux and is attracted to regions of a magnetic field where the field is stronger. On the other hand, a diamagnetic material, e.g. graphite, dilutes the magnetic field within its volume and is repelled from the stronger areas of magnetic field. The ultimate diamagnet, of course, is a superconductor, which rejects all of the magnetic field.

So here is how we could construct an artificial diamagnet. Start with a 3 dimensional Helmholtz coil, i.e., three mutually perpendicular sets of coils. These are available commercially, but I'm sure a useable one could easily be constructed. Inside put a 3-axis magnetic field sensor like the ones found in most smart phones, and a set of three amplifiers to drive the coils, with feedback so that the coils are energized until the three sensors see no magnetic field. So the assembly behaves like a diamagnet, generating a magnetic field which opposes any external field.

If this could be made small and light enough, perhaps it could levitate over a track made of refrigerator magnets. It would make an interesting toy.