3.5 Magnetic levitation Ë 67EMS is an attractive suspension with the on-board conventional electromagnets
suspended below a steel rail, thus providing an attractive force between the steel rail
and the vehicle. Since the suspension method is inherently unstable, the position
between the magnets and relation to the rail is monitored on a continuous basis and
active control is employed to ensure stability. The suspension achieves its maximum
lift in the stationary state, and at higher speeds, the lift force is degraded due to
induced eddy currents in the rail. EMS results in a small levitation clearance owing to
the relatively low field strength of the on-board electromagnet. In addition, it requires
feedback for stability and an on-board power supply system which gives extra volume
and weight, and also requires a high precision guideway due to small clearance.
EDS is the repulsive levitation in which superconducting magnets or PMs are used
on board. It employs the high magnetic field strengths of superconducting magnets to
achieve large levitation clearance and is self-stabilizing without feedback. If an on-
board PM is used, levitation clearance is smaller because the magnetic field strength
of the PM is far less than that of the superconducting magnet. These on-board magnets
with forward speed can produce eddy currents in the conducting guideway, and thus
produce levitation by repulsion. Since there is no lift while stationary, the vehicle is
supported by wheels at low speeds, and it will be like an aircraft takeoff as the speed
reaches a certain value. An updated EDS system, Maglev 2000 [14], adopts a high-
speed switch and quadrupole magnet technology, in order for the Maglev to be driven
at high speed along a single guideway and it may not slow down at the stopover
station. The track line only uses a double guideway in the station. This method is
possible to promote the EDS system engineering application at a lower building cost
of the train route.
The EDS and EMS systems require relative speed or active controls, Respectively,
for stable levitation. The mixed-휇(relative permeability) system is a combination
of different휇materials, namely ferromagnetic material (iron), diamagnetic material
(superconducting screen), and air (electric coil space). Therefore, the levitation device
is called mixed-휇system [49, 50]. An iron block is set inside the racetrack-shaped
exciting coils. A vertical-direction-restoring force is generated, but a horizontal direc-
tion is not stable due to the attractive magnetic force between each other. Once the
superconducting screens are installed between the iron block and coil, the attractive
force decreases because the strength of magnetic field is weakened near the screen
and the iron block can be stabilized horizontally. This system can keep a large air gap
between the iron block and the superconducting screens.
In addition to the above magnetic levitation, a strange magnetic suspension
phenomenon under the action of magnetic field also attracts the interest of scientists.
For example, frogs, grasshoppers, strawberries, and other diamagnetic organisms
may be suspended in a strong magnetic field environment [51].
In the early 1970s, the developments of Maglev transportation technology began
in Germany, the USA, Japan, Canada, and Great Britain. At that time, the USA, Japan,
and Canada often adopted a superconducting EDS scheme, while Great Britain used