5.2 Key components Ë 121The principles, methods, structure, functions, and specifications of the several
HTS Maglev measurement systems [14] will be discussed in detail in this chapter.
5.2 Key components
The basic structure and main function of any measurement methods and devices
generally include mechanical and electrical drive, control system, data collection
and processing, etc. However, HTS Maglev measurement devices need some special
requirements, for instance, the bottom wall thickness of the liquid nitrogen vessel
and various PMG. They are very important for the engineering applications of the HTS
Maglev vehicle.
5.2.1Permanent magnet guideway (PMG)
Figure 5.1 shows the cross-section drawings, the distribution of magnetic field lines,
magnetic field along the transverse directions, and magnetic field along the vertical
directions of the several PMGs. The parameters of NdFeB PMs are magnetic energy
product of 40 MOGe, coercivityHcof 979 kA/m, and relative permeability휇of 1.049.
The PMGs are composed of normal PMs and iron plates. The arrows represent magne-
tic poles where the arrowheads represent north. The first PMG in the table was used
in the first manned HTS Maglev test vehicle in the world.
The magnetic field in the center of the PMG is stronger than that at any other
position, and it decreases rapidly with the increasing of the gap from the surface of the
PMG. The surface magnetic field of the single pole normal PM is about 0.45 T, while the
surface concentrated magnetic flux density of the first PMG is up to 1.2 T. The magnetic
flux density is 0.4 T at 20 mm above the surface of the PMG and is about equivalent to
the surface magnetic field of a single PM.
5.2.2Liquid nitrogen vessel with thin bottom
We have developed a thin bottom liquid nitrogen vessel in order to realize real-time,
accurate measurements of Maglev performance of HTS bulk over PMG. This is a very
important technique in the HTS Maglev measurements over a PMG. Generally, the
wall thickness of liquid nitrogen vessel is not limited, and its main specification is
a low vapor rate. However, YBCO bulks are levitated over a PMG in the HTS Maglev
measurement system, therefore a liquid nitrogen vessel with a thin bottom is very
necessary in order to get a maximum net levitation gap, i.e. the gap between the
outside wall of the vessel and the surface of the guideway.