3.6 Permanent magnet Maglev Ë 69According to the Earnshaw’s theorem, PMs cannot suspend each other in stable
configuration. PMs cannot be entirely supported by other PMs alone; thus, at least one
degree of freedom must be eliminated by a mechanical constraint. In order to realize
the stable suspension, it is necessary to adopt special technology and ingenious
design. Although the levitation between two PMs itself is difficult, a combination
of PMs and electromagnets or superconductors can achieve stable magnetic levi-
tation. PM levitation can achieved by various design schemes: repulsive levitation
between both PMs with the same poles facing each other, PM levitated above a
DC electromagnet or superconductor and vice versa, attractive suspension between
PMs as moving components and DC electromagnets as fixed components, attractive
suspension between a combination of PMs and DC electromagnets and steel rails, etc.
The PMs levitated above a DC electromagnet are unstable in the lateral direction.
Wang and Ilene [60] have successfully demonstrated the stable repulsive levitation
method using PMs and air-core electromagnets. The mobile carrier PMs are levitated
based on the repulsive force above the oblong-shaped electromagnets, which constitu-
te one part of the guideways. The stability of the levitated PMs is regulated by another
part of the guideways. A state feedback controller with integral compensator is desi-
gned for the stability control. The PM levitation approach is confirmed successfully
by a silicon wafer transport in a microelectronic fabrication line. The levitation gap
of wafer carrier is 0.5 mm above the guideway surface, and the maximum velocity is
22.2 cm/s in a traveling range of 1.0 m.
Chen et al. [61] have demonstrated a larger moving range dual-axis repulsive sys-
tem using four active guideways. The feasibility and effectiveness of the Maglev system
have been clearly demonstrated experimentally. The features of the two-axis Maglev
guideway system include repulsive levitation, hybrid magnets, a passive carrier and
active guideway, an oblong coil, and a four-guideway design. This PM Maglev system
is similar to the system above, but its structure is more complex because the moving
range functions are at the dual axis.
In PM Maglev, generally, the weight of the vehicle is levitated by attractive forces
of PMs installed in the undercarriages of the vehicle. To control the attractive forces
of PMs, the levitation air gap between PMs and the guideway should be controlled by
active feedback. Tsunashima and Abe [62] have described the design and performance
of a mechanical air gap controller for a PM Maglev transport vehicle without active
feedback control. The mechanical air gap controller, consisting of a control lever
and small guide wheels which work as a fulcrum, can control the air gap within
required limits to levitate the vehicle. The effect of a dead zone in the mechanical
air gap controller influences the levitation performance. Field test results show that
the mechanical air gap controller can achieve 95% static levitation when the load
variation is less than±20% of the vehicle total weight, and stable levitation is achieved
by a control lever ratio that is larger than 3 [62].
Wei [63], in Shanghai Normal University, invented and developed a manned PMS
Maglev vehicle prototype and exhibited it in the 6th Shanghai International Industry