Magnetic levitation mystery

The latest Gadget Freak has a bit of a mystery to solve –

Nathan Loden’s Magnetic Levitation experiment started with the basic problem of stabilizing a frictionless position control loop with inherent metastability. A fast controller with phase-lead compensation took care of that. Then he discovered a strange phenomenon: The levitated object begins and continues oscillating in rotation about a vertical axis. Explanations have been proposed and discussed, but they remain theories. Can you reproduce his result and discover the cause? The apparatus consists of a wooden frame, a bolt with a coil, a power transistor, an infrared emitter and detector, a few op amps and passive components.

Magnetic levitation mystery – Link.