Wei-Kan Chu, Ph.D., Professor, UH
Ki Bui Ma, Ph.D., Research Associate Professor, UH
Thomas Wilson, Ph.D., JSC
Eunjeong Lee, Ph.D., Post-Doctoral Fellow, UH
THE HYBRID SUPERCONDUCTOR magnet bearing system for a lunar telescope is
based on passive magnetic levitation and the flux pinning effect of high temperature
superconductors. The rationale lies in the unique capability of high temperature
superconductors to adapt to the low temperature and vacuum environments of space or on the
moon and in their capacity to enhance system stability passively without power
consumption.
The superconducting state has two fundamental properties: zero resistivity and perfect diamagnetism.[1] Perfect diamagnetism, known as the Meissner effect, is the complete exclusion of magnetic flux from the interior of superconducting material. It occurs when a superconducting material is cooled in the presence of a magnetic field and has reached its superconducting transition temperature. Diamagnetism is the property that makes superconductors shield their interior from an applied magnetic field.
Dr. Thomas Wilson (l.), Dr. Wei-Kan Chu (l. center), Dr. Eunjeong Lee (r. center), and Dr. Ki Bui Ma (r.) comprise a joint UH-NASA team studying superconducting bearings for space telescopes.
In high temperature superconductors (also known as type II superconductors), however, any magnetic field already present would not be expelled, but would remain trapped inside the material.2 This flux pinning generates high stiffness to stabilize the position of the magnet, which is placed on a piece of high temperature superconductor with a certain gap and then cooled. The levitation force generated is, however, relatively small. For high temperature superconductors, superconducting state occurs below 90 K. The temperature of the lunar surface away from the sun can reach temperatures as low as 30 K-60 K.
Another advantage of the hybrid superconductor magnet bearing is its capability to prevent problems related to the use of conventional lubricants in cryogenic and vacuum environments such as freeze-up and evaporation. It also allows less tight tolerance in its manufacturing and assembly compared to conventional mechanical bearings. Tight tolerances common in conventional mechanical bearings can be readily clogged up with lunar dust. Thus, a superconducting bearing is an attractive replacement for a conventional bearing in the lunar telescope, which is stationed in cold and dusty lunar environments for an extended period of time.
The development of superconducting bearings for space telescope applications has been pursued in two parallel directions: the design of a new superconducting bearing system for the equatorial mount of a telescope on the moon and the characterization of an existing prototype superconducting bearing system designed for an azimuth mount.[2,3]
Azimuth Mount
In hybrid superconducting magnet bearing systems, levitation forces are produced mostly by
sets of two permanent magnets of the same or opposite polarity. However, there is an
inherent instability in the interaction of the two magnets (a result known as Earnshaw's
theorem). High temperature superconductors (HTSs) are used to enhance system stability by
pinning down the magnetic flux. The HTS-magnet interaction can also provide additional
levitation force.
Superconducting bearing systems experience nonlinear spring-like and hysteretic drag forces within some ranges of angular position as a consequence of the HTS-magnet interaction.[4,5] An experimental setup for characterization has been assembled to further the understanding of this dynamic behavior. This characterization will provide information for the design of future superconducting bearing systems, such as the drag force, its position dependence, stiffness, required motor torque, etc.
The experimental setup for the characterization requires several components. The superconducting bearing system is securely fixed on a Melles Griot solid aluminum optical breadboard. The platform of the telescope, which is attached on top of the bearing rotor, is coupled to a Himmelstein model MCRT 3L-08T, 10 oz-in range, non-contact rotating torque transducer via a Thomas miniature flexible disc coupling, which provides relatively high torsional stiffness along the shaft axis with high compliance in all of the five remaining degrees of freedom. The other shaft of the torquemeter is coupled to the MicroMo planetary gearhead 23/1 (gear ratio of 1526:1) via a Thomas miniature flexible disc coupling. The gearhead is screwed to a MicroMo brushless D.C. servomotor 2444S024BK315, which is screwed to a Hewlett Packard HEDS-5500 two channel incremental optical encoder with a resolution of 512 counts-per-revolution with quadrature output.
The torquemeter is connected to a model 61201DL universal strain gage amplifier and the measured torque is displayed through a model 61201-OZ-IN digital, universal strain gage amplifier-display. The brushless D.C. servomotor is connected to a low voltage pulse-width-modulation (PWM) amplifier, Western Servo Design BPW-S3-6/10.
For data acquisition and control, a motion control interface card for a PC, Precision MicroDynamics model MFIO-3A-ISA-0, is inserted into a PC. This I/O card has a three-channel encoder interface and DAC interface within one board with an open architecture allowing hardware access at the register level.
Mechanical supporting structures for the setup are designed and assembled. Computer codes were written for measurements. The integrated system for experiments is set up at a laboratory workspace at JSC in Building 268 for carrying out this investigation. The electronics need to be checked further for data acquisition.
Equatorial Mount
The design of a superconducting bearing for an equatorial telescope mount involves the
design and construction of the polar axis bearing and driving mechanism capable of
tracking stars to a precision of better than 0.25 arc secs. In order to track stars on the
moon, the motor should rotate the telescope platform at a constant velocity of 23 mRPM.
The polar axis is inclined with respect to the direction of gravity at an angle that
depends on the latitude of telescope deployment. Furthermore, a controller need be
developed and implemented to achieve such a high pointing accuracy.
Design Of Superconducting Bearing
The TCSUH design of a hybrid superconducting magnet bearing is based on the method which
utilizes the force of a uniform magnetic field on a single magnetic pole formed by a long
thin magnet. With this design the new superconducting bearing can support the polar axis
irrespective of its inclination angle, which is determined by geographical latitude. The
design of an actuator is under way that can be integrated with this bearing.
In its present form, the design can support one exact weight only. A certain amount of variance in the weight of the telescope to be supported will be incorporated into the next design stage by superposition of additional gradient magnetic fields.
Design Of Motor
System requirements present stringent demands such as high positioning accuracy (arc sec
resolution), low power consumption, non-contact and miniature design with light weight,
and full redundancy of coils and sensors. Along with these requirements, special
considerations have to be given to meet space qualifications in vacuum environments at
cryogenic temperatures.
In our design of the driving mechanism, a transmission is eliminated. This avoids undesired nonlinear dynamics such as stiction, Coulomb friction, nonlinear spring characteristics, and backlash.6 Direct drive also removes problems inherent in freeze-up and evaporation of lubricants in space environments.
A variety of actuators, including micro actuators, have been considered for integration with superconducting bearings. Electromagnetic forces were chosen since they allow non-contact action and their force/torque scale can match the physical requirements of our actuator system. D.C. brushless motors have been selected since they meet the low power dissipation requirement and also provide a precise control of torque/position.
Our conceptual design brings forth an ironless multi-pole D.C. brushless motor with three phases. It consists of two NdFeB permanent magnet ring rotors and a stator. Radial forces causing undesired disturbances are reduced by eliminating iron from the stator. The absence of iron in the stator also helps to significantly reduce the core losses, which occur when ferromagnetic materials are exposed to an alternating magnetic field. Two pairs of motors are used to guarantee redundancy. Three pairs of Hall effect sensors or infrared sensors are used for motor commutation. A pulse width modulation (PWM) amplifier is used for minimal power consumption and compactness. All the materials and procedures used meet space qualifications.
Design of Control System
A controller will be developed, based on the knowledge of hybrid superconductor magnet
bearing characteristics gained from the characterization experiments. The challenge lies
in maintaining the extremely low velocity of 2.42 mrad/sec (23 mRPM) against the unique
drag forces present at the hybrid superconductor magnet bearing.
Contributions
Both space programs and industry can benefit by the development of a superconducting
bearing using permanent magnets. Levitated superconducting bearings are free of friction
and decrease energy dissipation, power consumption, mechanical wear, and torque ripple,
thus resulting in long-life operation and less maintenance. With reduced system
nonlinearity, such as friction including stiction and torque ripple, design of control
system becomes easier and simpler, rendering more robust and high performance systems.
Another contribution to be made is a development of a driving mechanism, which enables extremely low velocity rotation (23 mRPM) with high resolution (less than 1 arc sec) without any transmission.
References
[1]P. J. Ford and G. A. Saunders. "High-Temperature Superconductivity-Ten Years
On," Contemp. Phys. 38.1 (1997):63-81.
[2]K. Ma, M. Lamb, P. Chen, T. Wilson, R. Cooley, H. Xia, C. Fowler, Q. Chen, and W. K.
Chu. "A Natural Application for High Temperature Superconductors: A Bearing for the
Azimuth Mount of a Lunar Telescope," Appl. Superconductivity 2.7/8 (1994):
479-86.
[3]M. Lamb, K. Ma, R. Cooley, D. Mackey, R. Meng, C. W. Chu, W. K. Chu, P. Chen, and T.
Wilson. "High Temperature Superconducting Bearings for Lunar Telescope Mounts,"
IEEE Trans. on Applied Superconductivity 5.2 (1995): 638-42.
[4]F. C. Moon, K.- C. Weng, and P. -Z. Chang. "Dynamic Magnetic Forces in
Superconducting Ceramics," J. Appl. Phys. 66.11 (1989): 5643-45.
[5]F. C. Moon, M. M . Yanoviak, and R. Ware. "Hysteretic Levitation Forces in
Superconducting Ceramics," Appl. Phys. Lett. 52.18 (1988): 1534-36.
[6]E. Lee, K. A. Loparo, and R. D. Quinn. "Hybrid Impedance/Time-Delay Control for
Robot Manipulators with Unknown Dynamics and Disturbances," Proc., ASME Int'l
Mechanical Engineering Congress and Exposition, Chicago, IL, Dynamic Systems and Control 1
(1994): 255-61.
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