Superconducting Bearings for Space Telescope Applications
Wei-Kan Chu, Ph.D., Professor, UH; Ki Bui Ma, Ph.D., Research Associate
Professor, UH; Thomas Wilson, Ph.D., JSC; Eunjeung Lee, Ph.D., Post-Doctoral Fellow, UH
The Earth's moon, with its
29 day-long rotation period (synodical) and having no atmosphere, presents an ideal
platform from which to observe the cosmos. A telescope there can accumulate light for 14.5
days or 350 hours, which is 460 times longer than the Hubble Space Telescope can perform
in Earth orbit. To take advantage of these conditions, efforts have been made to develop
ultra-light weight telescopes that could be stationed and function autonomously on the
Moon for an extended period of time. While lunar environment is favorable for observation
and study, however, it is unfavorable from the point of view of mechanical design because
of its extremely low temperature and the presence of electrically-charged dust on the
surface.
In the lunar environment, most bearings and gears, which are regarded as essential
elements in a conventional mechanical system design, are subject to failure induced by the
cold and dusty vacuum. Conventional lubricants in these bearings would either freeze in
the low environment temperature, or evaporate as a consequence of near-negligible vapor
pressure in the absence of an external atmosphere. Solar ultraviolet radiation may also
change lubricant performance. Furthermore, tight tolerances that are required for high
positioning accuracy would be susceptible to being clogged up with lunar dust material.
The accumulated dust particles make gear-teeth or bearing surfaces wear even faster, thus
reducing their life time. Moreover, gear-tooth wear introduces transmission nonlinearity.
While transmission (gear ) ratio must be a predictable constant, over the course of a
cycle the apparent transmission ratio can vary about this mean due to wear. Such
variations introduce cyclic accelerations of the transmission input and output, producing
an apparent "torque ripple." In fact, the effect is a position ripple. and the
resulting torque ripple depends on the dynamics of the transmission as well as the
dynamics of the loads attached to both transmission ports. Thus, position ripple should be
minimized, or, equivalently, transmission linearity should be maximized to minimize
resulting torque disturbances.[1] To avoid these difficulties, transmission can be removed
altogether by using direct-drive mechanism, whereby the motor is directly connected to the
output shaft without any speed reducer. A new actuating and drive system should be
developed which does not suffer from problems, previously discussed, encountered in lunar
application.
A terrestrial telescope operating in the lunar surroundings employs bearings to support
rotating parts of the pointing actuation system. In order to eliminate the disadvantages
of conventional bearings in lunar surroundings, a new bearing mechanism has been developed
by using hybrid superconductor magnet bearings (HSMBs). It has been used with an azimuth
mount of a telescope and shown to be stable, light, passive, and essentially
frictionless.[2]
Objective
The objective of this project is to design an equatorial mount for a telescope that will
use hybrid superconductor magnet bearings (HSMBs) on the polar axis to overcome
difficulties involved in lunar application and enable the telescope to track stars from
the Moon. The polar axis is the most crucial, since only counter-rotation about that axis
is required to follow a star across the celestial sphere, but this rotation has to be
maintained to a high degree of accuracy and precision. The HSMB can provide a very smooth
interface between the rotating shaft and the non-rotating frame to allow the desired
steady rotation to proceed under the control of properly designed system. In order to
utilize an HTS bearing for a lunar telescope, an actuation and control system needs to be
developed. To track stars, the actuator should be capable of rotating the platform at a
very low constant speed with high positioning accuracy. Therefore, this project focuses
upon two detailed objectives:
- design and construction of an actuator for HTS bearing rotor rotation;
- design and integration of a control system for the lunar telescope using an HTS bearing.
System Requirements
Lunar application places harsh demands on the design of a mounting system for a lunar
telescope. Requirements include the following characteristics:
- cryogenic operating temperature
(77 K);
- operation in a vacuum and an electrically-charged dusty environment;
- high positioning resolution
(~0.1 arcsec) and repeatability;
- low rotational speed
(~0.5 arcsec/sec).
Prototype HTS Bearing Module
The University of Houston, in collaboration with Prof. Chen, built the prototype HTS
bearing module to be assembled as an azimuth mount for a levitated lunar telescope. The
entire assembly has been observed to rotate continuously under its own inertia for over 45
minutes after being hand spun and rotated back and forth. Observations on the direction of
a laser reflected off a mirror mounted on the platform to an x-y position sensor were made
for various time scales.[2] A typical trace for the laser spot is shown in Fig. 1.
As described above,
the prototype oscillates about local equilibrium points exhibiting nonlinear damping and
spring characteristics. It seems to result from magnetic field unbalance attributed to the
manufacturing imperfections of magnets. These nonlinearities appear to be a few orders
larger than the required torques for stable rotation and should be carefully characterized
before the design of actuator and control system.
Methodology
First, a more compact and light-weight HTS bearing system will be developed with the
knowledge and experience obtained during the development of the prototype system.
Second, with an understanding of HTS bearing spring and damping characteristics, an
actuator to rotate the platform for lunar telescope will be developed. For our HTS
bearing, electrostatic force and electro-magnetic force appear to be the most suitable
actuating force. The design starts with modeling of the conceived design and is followed
by simulation. In order to investigate dynamic behaviors of the actuator, one dimensional
MATLAB simulation will be performed before two dimensional simulation with FEM (finite
element method).
Third, a sensing scheme will be devised and implemented. Redundant sensors will be
adopted to ensure successful operation even with a sensor failure.
Fourth, a control system will be designed to meet the system requirements. Considering
the autonomous operation on the Moon for long period of time, the control system should be
as simple and robust as possible.
Conclusion
Development of the fully integrated HTS bearing mount for the space telescope can be
achieved by synergistic integration of the mechanical design of the bearing module,
actuator design and control system design.
References
1W. S. Newman. "Comparative Evaluation of Three Transmission Types: Worm Gear,
Cone drive and Traction Drive," Technical Report 93109, Center for Automation and
Intelligent Systems Research, Case Western Reserve University, Cleveland, Ohio, 1993
2M. Lamb et al. "High Temperature Superconducting Bearings for Lunar Telescope
Mounts," IEEE Transactions on Applied Superconductivity, 5.2 (1995).
ISSO * 1995-1996 * Annual Report
