6.7 Guidance forces of HTS bulks over PMG Ë 167During the experiment, when the PMG was moving along thex-axis, the guidance
forces acting on the HTS bulks might make the vessel swing in thex-zplane. We rang
the vessel by a rigid epoxy ring with a little larger diameter than that of the vessel. Two
epoxy wedges inserted between the ring and the vessel, as shown in Fig. 6.18b. The
wedges prevented the motion of the vessel and the HTS bulks remained motionless
in the experiment. The guidance forces acting on the HTSCs were transferred to
the force transducer connected to the epoxy ring. The precision of the transducer
was±0.1 N.
The PMG was made of NdFeB and iron yokes, as shown in Fig. 6.18a. It was
controlled by a PC-based measurement system (see Sections 5.3 and 5.4); the velocity
was 1 mm/s. The displacement precision of the PMG was±0.1 mm.
The YBCO bulks were about 30 mm in diameter and 18 mm in thickness. The HTS
bulks were arranged into five arrays (Fig. 6.19) [34]. Thec-axis of each HTS bulk was
perpendicular to the top surface of the guideway. The HTS bulks were all field cooled.
The distance between the bottom surface of the HTS bulk and the top surface of the
PMG was 10 mm in both the cooling procedure and the measurements.
The guidance forces were sampled automatically as a function of lateral dis-
placement of the PMG during the experiment. First, the PMG moved in the positive
x-direction until the maximum lateral displacement (MLD) was achieved. Then, a
complete cycle was performed by moving the PMG in the negativex-direction for
a displacement of−2 MLD and reversing. Finally, the PMG returned to its original
position shown in Fig. 6.18. For each HTS bulk C or HTS bulks array, we measured
the guidance forces over lateral displacements with different MLDs, i.e. 0.5, 1, 2, 3, 4,
5, 7, 10, 12, 15, 17, and 20 mm. After all these MLDs were finished, the HTS bulk or the
Fig. 6.19:Arrangements of the HTS bulks. Of the four circles in each figure, those black and
numbered are HTSCs, with the rest epoxy bulks [34]. (a) Arrays 2, (b) 2, (c) 4, (d) 4, and (e) 5 and
(f) an example arrangement for a single HTS bulk measurement.