Magnetic levitation with PID control

The coils are made by winding wires to a core shown in this picture, which is made from a piece of pencil and two pieces of paper board.

There are two Hall sensors in the center of the four coils to detect the position of the floating magnet. There is a big ring magnet underneath the four coils (not seen from this picture since it under the wood board). The ring magnet provides the force to push the floating magnet up, however, it is not stable. The four coils stabilize the floating magnet in x and y directions.