8.2 Dynamic characteristics Ë 267vehicle system is designed to equilibrate with gravity and HTS bulk has strong flux-
pinning property, it may leave the equilibrium position for a new position when
disturbed. Six degrees of freedom as well as the coupled freedom may be the source
of a dynamic problem and there has few reports of dynamic stability of HTSC vehicle.
Figure 8.7 shows the vibration curves of points A, B, C, E, D, and F in the three
different directions. The forward speed is 56.6 mm/s and the FCH is 30 mm for the
initial position. It can be seen that the vehicle system has sufficient damping to ensure
free smooth running over the PMG. However, a better FCH still does not avoid existence
of slight dynamic motion.
Figure 8.7a shows the accelerometer signals in the forward direction. Points A, B,
and C have the same waveforms, and points E, D, and F have the same waveforms.
However, the initial phase difference of two groups is about 180°. From the viewpoint
of structure, the signal group with points A, B, and C means the right part of the vehicle
model while the other group means the left part. Obviously, the vehicle prefers moving
forward together with a swing motion. Moreover, the vibration curves are more like
random signals. This is because the present PMG is composed of many rectangular
magnets. The magnetism in homogeneity and air gap between each two magnets are
the main factors for friction. Once the left part and right part do not move forward at
the same time, the resistances have different effects for the two sides of the vehicle.
In the practical assembly of a PMG, the uniformity of the PMG should be guaranteed
as much as possible since it enhances frictionless movement of HTS Maglev vehicle.
Then, the swing motion can be very weak.
On the other hand, the coupled lateral vibration cannot be ignored because both
dynamic motions are in the same horizontal plane. Figure 8.7a and c indicates that
the vehicle model will produce a lateral-yaw dynamic. As shown in Fig. 8.7a, the
waveforms of points B and C or the waveforms of points D and F are the same but
with inverse phase. It is a typical lateral motion phenomenon. However, the back part
(points A and E) of the vehicle model have stronger lateral vibration because of the
damping effect of the vehicle model body.
As for the vertical dynamic motion in Fig. 8.7b, point A is almost the same as
points E and C is close to point F. The waveforms of the front part (points A and E) are
almost the inverse of the back part (points C and F) of the vehicle model. Vibrations
from the front and the back parts couple and counteract at points B and D, which
implies the existence of a pitch motion. When the speed increases, vibration also
happens periodically in the middle part of the vehicle. More investigation indicates
the trend of the pitch-heave dynamic that happens in the HTS Maglev vehicle system.
By combining all of the results from the vertical dynamics, it appears that the
levitation forces change with the slight lateral displacement when the vehicle model
moves forward with a lateral-yaw motion. The present position adjusts to another
new equilibrium. At a higher WH, the HTS Maglev vehicle model is a natural motion
because of the smaller effect of the magnetic flux density and its gradient. Compared
with Fig. 8.7a–c, the pitch-heave motion is stronger than the lateral-yaw motion,