Vibration Isolation for Space Station Microgravity Using a High
Temperature Superconductor and Magnet Interaction
Wei-Kan Chu, Ph.D., Professor, UH; Ki Bui Ma, Ph.D., Research Associate
Professor, UH; Thomas Wilson, Ph.D., JSC; Jang-Horng Yu, Ph.D., Post-Doctoral Fellow, UH
In this
project, we study the application of high temperature superconductors (HTS) as potentially
useful materials in the mechanical design of space structures. Much delicate equipment is
incorporated in individual satellites as well as onboard the space station requiring
isolation from vibration coming from sources that are incidental in some cases, but are
indispensable for the functioning of the entire unit in others. An example can be found in
the current proposed design of the Next Generation Space Telescope (NGST), consisting of a
collapsible truss structure connecting an infra-red telescope and a sun shield designed to
help keep the telescope itself sufficiently cold. The sun shield is a potential source of
undesirable vibrations due to temperature gradients across the thickness of the shield.
While the primary intent of the mechanical link between the telescope and the sun shield
is to keep the two together, the connection must be achieved without allowing too much of
the vibration generated at the sun shield to be transmitted to the telescope. In general,
it can be expected that forces transmitted in vibrations fluctuate more rapidly than those
forces required to prevent the telescope from drifting away from the sun shield
altogether. This can be accomplished by inserting a low pass acoustic filter in the
mechanical link between the sun shield and the telescope. For the purposes of the NGST,
the low pass cutoff frequency would have to be somewhat lower than 0.1 hz to provide a 60
db attenuation of vibrations with frequencies above 0.1 hz.
Basic Concepts
Consider an extreme case: all propagation of mechanical vibrations from the sun shield
towards the telescope can be blocked if the telescope is physically detached from the sun
shield. To prevent the telescope and the sun shield from drifting away from each other, we
need to re-establish a weak link whereby the telescope and the sun shield can still
interact via slowly varying forces and torques necessary to maintain, in the long run, the
relative position and orientation of the telescope with respect to the sun shield. We
visualize that such interaction to be mediated by forces that act at a distance, so that
direct physical contact which can generate and transmit high frequency mechanical
vibrations readily can be eliminated. A natural candidate is the flux pinning force
between permanent magnets and HTSs that has a tendency to maintain any spatial gap
initially set between them when the superconductor last turns superconducting.
A crude device can be made with a permanent magnet attached to the sun shield and a
piece of HTS attached to the telescope, but the sun shield and the telescope are otherwise
completely detached from each other. The temperature of the HTS is to be kept above
critical (90 K) until deployment, when it is to be brought into the vicinity of, but not
in contact with the permanent magnet attached to the sun shield, and allowed to enter into
the superconducting phase as its temperature drops to that at which the telescope is
designed to operate, which is somewhere between 60 K on the sun shield side and 30 K at
the telescope. Under such conditions, the motion of the telescope with respect to the sun
shield is expected to behave as if there were an invisible spring tethered between the two
otherwise detached components. Vibrations with frequencies higher than the natural
frequency of vibration of this invisible spring connection will not be effectively
transmitted between the sun shield and the telescope.
The device outlined above is passive, operating without a power supply. This represents
a great advantage for missions with a tight energy budget. It is also anticipated that
this device will occupy a very small fraction of the total mass and volume of its payload.
The device is extremely simple, making it less likely to fail unexpectedly. The device is
based upon a working principle that is fairly robust and hence tolerant of minor errors.
By the same token, it is almost maintenance free.
Directives
In this study, we want to characterize the magneto-mechanical properties of the flux
pinning force between a HTS and a permanent magnet, particularly in regard to the
transmission of mechanical vibrations from the superconductor to the magnet, or vice
versa. During the course of this study, we will identify and determine the critical
parameters necessary for the optimal design of a superconductor magnet pair for vibration
isolation purposes. For instance, it was mentioned earlier that, when the HTS interacts
with a permanent magnet, the magnetic field behaves as an invisible cushion between the
HTS and the magnet. Such a magnetic field exhibits some of the properties of a
viscoelastic material which, in a preliminary modeling, can be represented in the
mechanical form of a spring and a dashpot. The stiffness of the equivalent spring and the
damping coefficient of the effective dashpot, and, more importantly, conditions that
govern these quantities, would be invaluable information input to the design of vibration
isolation elements using magnets and HTSs. Moreover, the spring exhibits a nonlinear
strain hardening phenomenon (Fig. 1) and has a very low stiffness
plateau for a wide range of deflection. In this flat plateau, since the equivalent spring
stiffness is small, the mechanical system will exhibit a low natural frequency close to 0
hz. Such a phenomenon can be advantageous when used in the design of a vibration isolator
that will effectively attenuate vibrations with frequencies greater that 0.1 hz. Finding a
way to convert this idea from concept to practice is an important component of this piece
of research.
Approach
So far, we have identified two different approaches capable of producing configurations of
magnets and HTSs that are potentially useful as basic building blocks for a vibration
isolation system. In general, when a permanent magnet approaches an HTS, the stiffness of
the repulsive force rises steeply in a manner reminiscent of the way the stiffness
increases as two permanent magnets approach each other. In the case of permanent magnets,
this rise of the stiffness can be softened considerably if a hole-preferably large enough
for one magnet to go through-is made in one of the magnets. It is surmised here that a
similar tendency would occur with magnets and superconductors; specifically, designs
consisting of a composite magnet bar and a ring made of HTS (see Fig. 2), or a bar of HTS
in a magnet ring (see Fig. 3), are promising candidates with good vibration isolation
properties. The other approach makes use of the fact that an HTS is free to move about in
a uniform magnetic field but is restrained by a gradient of the magnetic field. In
practice, a uniform magnetic field can only have a finite extent, and so an HTS that finds
itself within such a volume would be able to move without an opposing force only within
the volume in which the magnetic field is uniform, but would experience a restraining
force as it wanders outside that region. Since the field inside a magnet ring with
appropriate aspect ratios can be quite uniform, the configuration of a bar of HTS in a
magnet ring (see Fig. 3) mentioned above is also suggested by this approach. Another
configuration that follows from this approach is an HTS disc between two magnetic pole
pieces (see Fig. 4), or one magnetic disc between two HTS pucks (see Fig. 5).
Methodology
For each of these potentially interesting configurations, it is important to know its
dynamic response and the resonance frequency of relative motion for vibration isolation
design. We propose the following schematic experimental setup (Fig. 6)
for measuring the dynamic response. Each of these configurations consists of a magnet
piece and an HTS piece. One of these pieces is attached to a block of structural support
material with an accelerometer attached, while the other piece is attached to a separate
block of structural support material with a shaker attached. The amount of vibration
transmitted from the shaker to the accelerometer through the magnetic flux linkage passing
from the permanent magnet to the superconductor describes the dynamic response that we are
looking for. However, in order to carry out this measurement under the gravity of Earth,
we have to assess and make allowance for the effects of any mechanical support which we
must provide to the separate blocks of material.
Furthermore, from previous
experiments, we know that the mechanical response of the equivalent spring is, in fact,
significantly nonlinear and hysteretic. Therefore, in a more realistic approach, it would
be necessary to take the nonlinearity of the spring into account, and to provide a
relaxation model to describe the hysteresis phenomena in the force response curve. This
activity would have to be supported by systematic measurements of the amplitude dependence
of the vibration transfer function for the pair of magnet and HTS involved. With a
comprehensive understanding of the behavior for one pair of magnet and HTS, we can push
forward to examine how these may be installed in a network integrated into the truss
structure between the telescope and the sun shield to provide for an adequate level of
vibration isolation (60 db down, or an attenuation factor of 1000).
Summary
To summarize, we can subsume our research program into the following five objectives:
- to identify configurations of HTS magnet pairs that have desirable vibration isolation
properties;
- to characterize the vibrational properties of these HTS magnet pairs in terms of dynamic
stiffness, damping coefficients, and transfer functions;
- to construct descriptive models for the vibrational properties of these HTS magnet pairs
and to improve upon these models to include nonlinearity and hysteresis;
- to examine the vibration isolation properties of the above vibration isolation elements
when incorporated into a network;
- to construct prototypes of vibration isolation devices for possible application in NGST
and on board Space Station Freedom.
Conclusion
HTS materials have shown great promise for use in vibration isolation devices in space
structure applications. However, before good engineering designs for these applications
can be derived, a deeper understanding of the dynamic response between HTS materials and
magnets must be attained. The success finding of this study will contribute significantly
to space technology.
Acknowledgement
We are grateful to Dr. Peter Chen of Computer Sciences Corporation and NASA, Goddard for
sharing with us information on the NGST.
ISSO * 1995-1996 * Annual Report
