A bearing for telescope mounts on the moon has to function in a cold dusty vacuum environment that impairs the operation of almost all traditional bearings, but it is a natural environment for bearings constructed out of magnets and high temperature superconductors. The challenge lies not so much in the weight of the telescope that has to be supported, but in the smoothness of forces required for precision positioning control over a long stretch of time without human intervention. In this paper, we present a design of hybrid superconductor magnet bearings intended for use on the azimuth mount of an altitude-azimuth telescope mount system. In addition to the general features of hybrid super conducting magnet bearings, we will address particular issues connected with the application of these bearings on a telescope mount

Chen, Peter

Chen, Quark

Chu, Wei-Kan

Cooley, Rodger

Fowler, Clay

Lamb, Mark

Ma, Ki

Wilson, Thomas

Xia, Harold

English

NASA Technical Reports Server

NASA-TM-112487
Pergamon
, -_ . . -//;,
I
Applied Superconductlt'ttv Vol 2. No. 7,8. pp 479-486. 1994
Copyright 1995 Elsevier Soence Lid
0964-1807(94)00040-9 Pnmect m Great Britain. All rights reserved
0964-1807/95 $9.50 + o.0o
A NATURAL APPLICATION FOR HIGH TEMPERATURE
SUPERCONDUCTORS: A BEARING FOR THE AZIMUTH
MOUNT OF A LUNAR TELESCOPE
KI MA, _ MARK LAMB, t PETER CHEN, 2 THOMAS WILSON, 3 RODGER COOLEY, 1
HAROLD XIA, t CLAY FOWLER, t QUARK CHEN t and WEI-KAN CHU'
*Texas Center for Superconductivity and Department of Physics. University of Houston.
Houston, TX 77204-5932,
-'Computer Sciences Corporation. Calverton. MD 20705, JJohnson Space Center,
National Aeronauttcs and Space AdministraDon, Houston, TX 77058-3696. U S.A
Abstract--A bearing for telescope mounts on the moon has to function in a cold dusty vacuum
environment that Lrnpairs the operatton of almost all traditional bearings, but it is a natural environment
for bearings constructed out of magnets and high temperature superconductors. The challenge lies not so
much in the weight of the telescope that has to be supported, but in the smoothness of forces required
for precision positioning control over a long stretch of time without human intervenuon. In this paper,
we present a design of hybrid superconductor magnet bearings intended for use on the azimuth mount
of an altitude-azimuth telescope mount system. In addition to the general features of hybrid super
conducting magnet beanngs, we will address particular issues connected with the application of these
bearings on a telescope mount•
INTRODUCTION
:"_he moon has long been a coveted site for an observatory with the astronomers. It offers cold,
dark vacuum conditions of interplanetary space combined with the stability of a massive anchor.
Together with the 27-day rotation period of the moon, it allows the astronomer to have long
continuous periods of observation under viewing conditions free from atmospheric disturbances
and other sources of interference.
However, the moon has been quite inaccessible until this century. Even now, visits to the moon
can only be infrequent. Moreover, the payload that can be carried there is limited to about 200 kg.
This implies that telescopes to be stationed on the moon must be extremely lightweight, and able
to. function by itself for long periods of time (estimated at 5 years) without on site human
maintenance. The mass of the telescope assembly and structure can be minimized by using materials
and fabrication technology proven in space applications, to produce precision optical elements of
very low a_eai density (< 2 kg/m 2) [1].
While the lunar environment is ideal for observations, it is harsh on the equipment with at least
two distinct components, the electronics involved in imaging and the mechanics of the bearings
on the telescope mounts. The difficulty with the electronic component is being overcome with a
new generation of ultraviolet detectors using charge-injection devices [2]. Here, we propose to
utilize superconductor magnet bearings to circumvent difficulties associated with the mechanics of
the mount.
Existing machinery that can achieve the kind of extremely fine motion required to track a star
from the moon are massive, require carefully controlled environments, and need periodic main-
tenance, rendering them unsuitable for use on the moon. Moreover, the low temperature of the
lunar surface requires the use of solid lubricants, which suffer from torque noise problems.
On the other hand, this low temperature is sufficiently low for the recently discovered high
temperature ceramic superconductors (HTSs) to work with only minimal amount of thermal
shielding, without the use of cryogens. One of the earliest suggestions for application of this novel
material was in levitation magnet bearings [3-6]. Such superconductor magnet bearings would
operate naturally in the cold vacuum environment on the moon if due care is taken to protect the
telescope from the Sun's heat, either with heat shields or placed in a deep crater at the poles. These
bearings exhibit multi-axts stability and they are passive (i.e. require no power consumption for
operationL There is no mechanical contact between the rotor and stator of the bearings, and hence
479
https://ntrs.nasa.gov/search.jsp?R=19970021185 2020-06-16T02:29:45+00:00Z
480 KIMAet ul.
do not wear out with use and require no maintenance. For the same reason, they are essentially
frictionless, permitting electronic positioning control with exceedingly fine resolution and an
extended range of motion.
Besides being cold, the lunar surface is also constantly bombarded by particle radiation from
solar flares and the solar wind. These charges up lunar dust particles, causing them to jump
sometimes as high as 1 m across the terminator from the light to the dark side. Our bearing is
tolerant of such dust storms, since there is a wide gap between the rotor and the stator. One might
also be concerned with the effect of such particle radiation on the electromagnetic property of the
HTS itself. The crucial electromagnetic property of the HTS material for holding the bearing
together is its critical current density. As it turns out, the cumulative effects of 5 years of exposure
on the lunar surface can be expected to improve the critical current density slightly [7, 8], and
hence should not constitute an impediment to the performance of the bearing during this time
period.
HTS/MAGNET INTERACTION
There has been a long history in the effort t_ exploit magnetic _forces to create noncontact
bearings. Magnetic bearings invariably incorpqrate active coot/rol circuits because a system
consisting of permanent magnets and/or-ferromagnetic or paramagment materials is inherently
unstable, a result embodied in Earnshaw's theorem [9]. Indeed, the active magnetic bearing is a
highly developed and sophisticated device, k_
Stable configurations become possible if we introduce diamagnetic materials. A case in point is
that of a permanent bar magnet, levitated above a hemispheric:_i bowl of lead at liquid helium
temperatures [10]. Here, lead, being a type I superconductor, _-'haves as a perfect diamagnet.
Unlike lead, the HTSs are type II superconductors. Their behavtor is more akin to that of an
almost perfect conductor than that of a perfect diamagnet. A perfect conductor has a negative
susceptibility towards differential increments in a magnetic field, just as a perfect diamagnet.
However, any magnetic field already present would not be expelled, but would remain trapped in
a perfect conductor. The implication for magneto-mechanics is that the levitation force on a magnet
placed on a piece of HTS material which is then cooled is small, but the stiffness, which stabilizes
the position of the magnet, remains appreciable. " - " -- --
HYBRID SUPERCONDUCTOR MAGNET BEARING
The above prediction has been demonstrated by actual experiment. We can construct a magnetic
bearing in which active control is replaced by the stabilizing action from a HTS. The HTS is cooled
after being assembled into the bearing, and is neither expected nor required to provide any thrust.
This is the essential concept of the hybrid superconductor magnet bearing [4, 5].
To function as a bearing, a rotor must rotate freely about an axis with respect to a stator, it is
important to note that, although the magnetic flux penetrating the volume of the HTS is strongly
pinned, this force is not transmitted outside the HTS, and in particular, not to the permanent
magnet although they are linked via the magnet flux lines. The analogy xvith a conductor suggests
that it is a change in the magnetic field that the HTS responds to. If the magnets on the rotor are
azimuthally symmetric about the rotation axis, the rotational motion of the rotor magnets do not
incur any change of magnetic field in the HTS. Hence, there will be no drag force against the rotation
of the rotor magnets. This has also been directly verified by experiments [11].
In practice, it is often found that the stabilization that the HTS can provide is not quite sufficient
to overcome the instability of the magnets. We have employed two approaches to make up for
this. One approach enhances the stability with high gradients of the magnetic field "'frozen" into
the HTS. This is easy to understand as the converse of the argument in the last paragraph. A higher
gradient of the magnetic ticld results in a larger change in the magncuc licld in the superconductor
for a given displacement that produces a stronger restoring force.
The other attempts to stabilize the magnet system itself. This can be _lchieved most readily b_
using s._mmetry to balance the instabilities offeach other. For a system t)l rolt)r ;tnd stator magnetls
HTSC_ una a lundr lClC'_COpC 481
with cylindrical symmetry and opposite parity li.c. if B_-r) = Blr) for the rotor magnet system,
then i_-rl = -B(r) for the stator magnet systemJ, it can be shov_n that the stiffnesses in all the
translational and rotational directions vanish, but that a finite thrust remains on the rotor from
the stator. With further refinement, we can proceed one step further to eliminate cross coupling
between translation in a direction perpendtcular to the symmetry axis and rotation about the axis
perpendicular to both. This creates a system with a neutral equilibrium that is, in a certain sense,
the most stable system that can be achieved without violating Earnshaw's theorem. In practice, a
small instability always remains, due to imperfections. These should be small enough to be rendered
insignificant by the action of the HTS.
!
/
/
I
I
/
!
/
Fig. I. Three-dimensional o_,erv_ew of bearing assemtHv
APSUP Z:T/B-D
482 K! MA et al.
BEARING FOR LUNAR TELESCOPE
The bearing on the azimuth mount of a lightweight telescope on the moon need sup'port only a
very light load, but stability is still essential. On earth, this is achieved by using counterweights,
so that the center of gravity of the telescope assembly falls below the pivotal point of support. This
entails additional mass to be added to the telescope assembly, which is undesirable for a telescope
to be transported to the moon. Our bearing design takes care of this problem by providing an
effective counterweight with magnetic forces in t:_e same way that these were utilized to support
the weight of the telescope assembly itself. A three-dimensional overview of the bearing is presented
in Fig. 1. A cross-sectional view is shown in Fig. 2 and a schematic of the magnets and HTSs
used in the bearing is displayed in Fig. 3.
As can be visualized from these figures, our bearing design consists of a pair of bearings with
opposite thrusts. The bottom bearing provides a downward thrust, and acts as the counterweight.
The top bearing provides the thrust that is the sum of this counterweight and the a,ctual weight of
HTSCs and a lunar telescope 483
(o'_o V"
i
N S
i denotes high
temperature
• /
t superconductors
,f i
both denote
permanent
_1] magnets
/SN
Fig. 3. Schematic diagram showing the magnets and superconductor comprising the bearings.
the telescope assembly. Each of these bearings is designed with neutral stability. The rotor magnet
consists of two concentric ring magnets, one fitted into the other with opposite polarity. The
magnets were designed to strengthen the stability obtainable from the HTSs by maximizing the
gradient of the magnetic field that would be frozen into the superconductors. However, the opposite
polarity arrangement also implies that the thrust that can be obtained from this volume of magnetic
material is just the net of two opposing thrusts, and is not the optimal. We can afford to do this
here, as the weight to be supported is not excessive. The stator magnets are pairs of cylindrical
magnets with a center hole, placed symmetrically above and below the corresponding rotor magnets.
The stator magnets of the top bearing is taller to provide a higher thrust. The distance between
the stator magnets in each of these pairs can be adjusted without destroying the symmetry of the
positioning with respect to the corresponding rotor magnet. This allows the thrusts provided by
these bearings to cover a finite range without giving up the condition of neutral stability for the
magnets.
484 KI MA et ul.
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(c)
10
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o ,'7¢>
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-5
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Fig 4 Traces ol" laser spot angular posnlons for ob,,ervatnon permds of (a} 8 ms. Ibl 70 ms. fC}05 s, Idl
S s and 4el 32 _. respectnvely.
HTSCs and a lunar telescope 485
PERFORMANCE OF BEARING
The top bearing can support an axial load of 45 N. It has been used to support a model telescope
with a weight of 12 N, the equivalent counterweight of 12 N from the bottom bearing, and the weight
of the levitated structure itself making up the remainder. The entire assembly has been observed
to rotate continuously under its own inertia for over 45 rain. The bearing assembly excluding the
telescope is about 1 m tall. The top plate is 11.4 cm in diameter, while the base spans 56 cm. The
central shaft is slightly less than 7.6 cm in diameter. The levitated structure is held in place with a
total stiffness from the bearings of about 10 N/mm in the axial direction. This low stiffness will be
supplanted by the stiffness due to feedback control in the complete system. The HTSs are enclosed
in two structures 7.6 cm tall and 16.5 cm in diameter, made of Styrofoam and G-10 glass fibre/epoxy
composite and fixed separately at 25 and 84 cm from the top. With all the necessary fixtures
included, the whole bearing assembly weighs 67 N.
To the astronomer, an important performance pa-ameter is the stability of the bearing. In order
to determine this, we mounted a mirror on the bearing and directed a laser beam to be reflected
from the mirror onto an x-.v position sensor. We monitored the position of the beam on the x-y
position sensor for continuous time periods of different lengths, from 8 ms to 32 s. Traces of the
laser spot on the sensor are reproduced in Fig. 4. The accumulated spot size, defined by the standard
deviation of the angular position from the average over the entire observation period, is plotted
as a function of the observation period in Fig. 5. We can see that the spot size increases from about
0.97/_rad of arc at 8 ins of time to around 19.4/_rad of arc after 32 s of time. For observations
from the moon, an accuracy of pointing to 0.48/_rad of arc is desirable. The angular measures were
converted from the position on the x-y position sensor by dividing over the distance (3 m) from
the mirror to the x-v position sensor.
Before a telescope is fully functional, other parts, most notably the drive mechanism itself and
_ts ass(_ciated control, must be de,_etoped, and then integrated into the system. What we have shown
is that this control motor mechanism that is eventually responsible for tracking stars would have
to be able to act within a time frame shorter than 8 ms. The bearing itself can use a shorter height.
Horizontal and tilted versions await development, as these would be needed in the altitude or
equatorial mounts and lifetime and reliability tests must be performed.
101 --
Spot size vs observation period
1o o
I0-1 1 I I I IIII] _ I I I III1[ 1 I I I 1111_ 1 1 I I tllll I I I I IIII]
10-3 10-2 10-1 IO0 10 | 102
Time (s)
Fig. 5. Spot size las measured by the standard deviation from mean positiionl as a function of observauon
period.
486 Kt MA et al.
SUMMARY
We have demonstrated that the hybrid superconductor magnet bearing holds promise as a viable
solution to the problem of fabricating a bearing for the exceedingly fine drive mechanism required
for a lunar telescope.
Although developed for use on the moon, these bearings could conceivably be applied on earth
as well, provided that a cold vacuum environment is created for their accommodation. In some
applications, such as cryogenic turbopumps, such an environment may already exist for other
reasons. In others, such as flywheels for kinetic energy storage [I 1], the inconvenience of having
to maintain a cold vacuum might be acceptable as a small price for a large gain in performance.
We hope that these bearings will find themselves useful and be of service in numerous applications,
on the moon, in space as well as on earth.
Acknowledgements--This work is supported in part by the Advanced Research Project Agency and in part by the State of
Texas through the Texas Center of Superconductivity at the University of Houston. The collaboration and hospitality of
R. J. Oliversen at NASA/Goddard Space Flight Center, H. Hojaji at Catholic University of America and R. E. Pitts at
Computer Sciences Corporation is gratefully acknowledged. We are also indebted to excellent suggestions from Chase
McMichael at the University of Houston.
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A Natural Application for High Temperature Superconductors: a Bearing for the Azimuth Mount of a Lunar Telescope

A Natural Application for High Temperature Superconductors: a Bearing for the Azimuth Mount of a Lunar Telescope

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