electromagnetic suspension was, proposed by Graeminger in the 1930's.
In the 1970's, Germany started development of this approach, which
has resulted in the Transrapid System (see Maglev
in Germany). The suspension is inherently unstable because,
as the gap between the electromagnet and the iron sheet decreases,
the attractive force increases. However, the suspension can operate
stably by continuous servo control of the winding current in the
electromagnet. If the gap between the vehicle electromagnet and
the guideway iron sheet above it decreases, the current in the winding
on the electromagnet is reduced, reducing the attractive force.
If the gap between increases, the current is increased. This current
is servo controlled on a time scale of a few thousandths of seconds.
The electromagnetic suspension was, proposed by Graeminger in the
1930's. In the 1970's,
Although the electromagnet approach eliminates the high cost of
permanent magnets on the guideway, the problem of very small gap
between the vehicle and the guideway remains. In the Transrapid
System, the gap is 3/8th of an inch. Because of the narrow gap,
the guideway construction and maintenance tolerances are extremely
tight which substantially increases cost. In addition, ensuring
adequate clearance under various environmental challenges, such
as ice and snow buildup, earth settling, earthquake movements, and
thermal expansion due to temperature changes is difficult.