The EMS attractive levitation system is shown in Fig. 5. Electromagnets (called support magnets) are attached to the train and powered by batteries on the train. There are coils (called stators) built into the lower surface of the track (called the guideway). The coils are comprised of sheets of steel and coil windings. When the electromagnets are switched on, the attractive force between the electromagnets and the coils levitates the train. Guidance electromagnets are located on the side to keep the train in position laterally.
To stabilize the levitation, a feedback system that utilizes gap sensors keeps the gaps between the train and the guideway at prescribed width. The gap sensors attached to the train contain oscillatory circuits that induce eddy currents in the stators under the guideway. When the gap width changes, mutual electromagnetic induction between the gap sensors and the stators will produce characteristic signals in the oscillatory circuit. The signals are analyzed and used to regulate the power delivered to the electromagnets of the train, maintaining the prescribed gap width. In engineering, this kind of control is known as feedback control. One can find toys that achieve stable magnetic levitation using feedback control (Fig. 6). Look at the video clip below: