Vibration Isolation for the Space Station and Vibration Isolation for Space Structures Using HTS-Magnet Interaction

WE ARE INVESTIGATING A PASSIVE VIBRATION ISOLATION DEVICE employing HTS-magnet interaction. The configuration of the vibration isolator consists of a ring magnet and a thin disk, or ringed HTS where the HTS is located in the middle of the magnet and is levitated. Experiments show that the natural frequency of the system is 4 Hz and that frequencies above 10 Hz are successfully isolated. Such a passive device in space applications is superior to similar active devices that often require bulky control circuit boxes and consume considerable energy not readily available in the space environment. The concept can also be used as an isolation platform and can combine with active vibration isolation technology so as to attenuate the vibration of all frequencies.

Objective
To design a vibration isolation device under the condition of microgravity is the objective of the task force. The requirement for such a design is low power consumption, sub-Hertz vibration attenuation, compactness, and reduced weight.

Methodology
Our design concept seeks to establish a weak coupling between the main structure and the vibration isolated object. This weak coupling is provided by the interactions between the high temperature superconductor and permanent magnets.

Concept
High-T_c superconductors (HTS) with high critical current density can be employed as elements of devices where a superconductor stably levitates above a permanent magnet. Vibration isolation systems, where the levitation phenomenon is used, are promising applications of the high-T_c superconductors. A numerical method has been derived for controlling vertical vibrations of a levitated high-T_c superconducting body.¹ The modeling and control of the vertical vibrations of the HTS levitation system, subjected to external disturbances, are discussed in Nagaya et al.² The following data study the horizontal vibrations of levitated superconductors above a ring permanent magnet.

Experimentation
Our investigation utilized samples prepared by the melt-texture method together with a seeded directional solidification method.³ The superconducting disk had a diameter of 40 mm and a height of 20 mm. The superconducting ring had an outside diameter of 40 mm, inside diameter of 16 mm and height of 20 mm. The critical current density of these samples, measured by the four-contact impulse method for a small specimen cut from bulk, was on the order of 10⁴A/cm² at 77K.

Fig. 1. The principal scheme of the experimental setup.

A schematic diagram of the experiment is shown in Fig. 1. A superconductor is mounted in a heat insulator cup above a permanent magnet attached to a table equipped with a shaker. The shaker generates horizontal vibrations. The superconductors face a magnet with an initial gap before the superconductors are cooled below T_c using liquid nitrogen. The superconductors are stable, levitating at the actual gap without any supports during the measurement. An accelerometer is set up on the heat insulator cup and on the magnet to sense the vibrations.

The signal from the accelerometer is sent to the Dynamic signal analyzer for analysis. Vibration transmissibility can thus be obtained. The magnet can be described as a ring-shaped Nd-Fe-B with an inner diameter of 51 mm and an outer diameter of 70 mm, and a thickness of 13 mm. The surface field was characterized at 0.47T.

Fig. 2. Transmissibility as a function of frequency of external disturbance.

Results
The horizontal vibration transmissibility for the superconducting disk(square symbols) and ring (circle symbols) are shown in Fig. 2. An initial gap was about 2 mm for the ring and 3 mm for the disk. For comparison, we give the transmissibility of the ring magnet attached to the shaker (triangle symbols). According to Fig. 1, a few resonance peaks are found for the horizontal vibration transmissibility. Sharp resonance is a characteristic at the natural frequency about 4 Hz for both the superconducting rim and disk. The observation of several peaks is the result of co-existence of certain modes of vibration in the horizontal vibration measurement. For instance, the first mode is attributed to the levitated superconductor horizontally vibrating. The second mode is attributed to the superconductor rotationally vibrating around its vertical axis. The transmissibility of the shaker is higher than that for the levitated superconductors. This activity means that the system, with the levitated superconductor above the ring magnet, can be used as the vibration isolation system. If the driven external force, which is proportional to the voltage from the generator, increases (see Fig. 1), the amplitude of the horizontal vibration also increases. The damping factor k for the first mode is defined as the width of the biggest resonance peak divided by 1/ \|2 and is practically constant. In our case, this factor is 4.4 Hz.

Fig. 3. Response of the accelerometer as a function of time at free damping vibrations.

The damping factor was also determined for the levitated superconductor at free vibrations. Using a pulse-function generator with an impulse width of 100 ms, we observed free vibrations of the levitated superconductors. The typical response of the accelerometer attached to the superconductor is shown in Fig. 3. Assume that a vibration mode of the horizontal motion is more excited than any other mode. In this case, the system has damping oscillations, which are changed in time as x = A₀e^-ki sin(wt + j₀), where A₀ and T₀ are constants. A = A₀e^-kt is the amplitude of the damping oscillations.

From experimental data of free damping vibrations, k is equal to 4.3 Hz for the disk and 4.1 Hz for the ring. The damping coefficient is c = 2mk. In our case, the mass m of the disk is 0.136 kg and the ring, 0.101. Thus, c for the disk is equal to 1.16 Ns/m, and c for the ring is equal to 0.83 Ns/m.

Conclusion
Horizontal vibrations of the system have been investigated where the superconductor (disk or ring) is levitated over the ring magnet. The resonant frequency and the damping coefficient of such a system have been found, based on experimental data of transmissibility. Resonant frequency of 4 Hz was achieved for the horizontal vibration comparable to passive vibration isolation systems.

Study of the damping coefficient of the system indicates that the levitated superconductor above the ring magnet is not large enough for real application; additional damping is required for active vibration isolation.

References
¹K. Nagaya, "Analysis of a High-T_c Superconducting Levitation System with Vibration Isolation Control," IEEE Trans. Magn. 32 (1996): 445-52.
²K. Nagaya, M. Tsukagoshi, and Y. Kosugi. "Vibration Control for a High-T_c Superconducting Non-Linear Levitation System," J. of Sound and Vibration 208 (1997): 299-311.
³D. F. Lee, C. S. Partsinevelos, R. G. Presswood, Jr., and K. Salama. "Melt Texturing of Preferentially Aligned Y-Ba-Cu-O Superconductor by a Seeded Directional Solidification Method," J. Appl. Phys. 76 (1994): 603-5.

Presentations

SUPERCONDUCTIVITY—Ki Bui Ma, research associate professor, operates an oscilloscope utilized in space applications. The development of superconducting bearings is a major achievement for space components. Superconducting bearings are suspended by magnetization, thus reducing friction on mechanical components. Conventional bearings would be clogged by lunar dust. Superconducting bearings avoid the problems one faces with lubricants in cold vacuum conditions on the Moon.

Ma, K. "Applications of High Temperature Superconductors in Magnetic Bearings and Flywheels," NATO Advanced Study Inst. on Applications of Superconductivity, Loen, Norway, Aug. 1997.
W.-K. Chu. "Application of Bulk YBCO Levitation Bearing for Flywheel and Vibration Isolation Device," invited talk, Int'l Workshop on the Processing and Applications of Superconducting (RE)BCO Large Grain Materials, Fitzwilliam College and IRC in Superconductivity, Univ. of Cambridge, United Kingdom, July 7-9, 1997.
--. "Applications of High Temperature Superconductors for Magneto-Mechanical Devices in Space," invited talk, TMS Annual Mtg., San Antonio, TX, Feb. 15-19, 1998.
--. "Bulk Applications: Overview," invited talk, 1997 Int'l Workshop on Superconductivity sponsored by ISTEC and MRS, The Big Island, HI, June 15-18, 1997.
--. "High Temperature Superconductor Levitation Bearings and Applications," invited talk, Humboldt Univ. of Berlin, Berlin, Germany, July 16, 1997.
--. "High Temperature Superconductor Levitation Devices," invited talk, 3rd Int'l Summer School on High Temperature Superconductivity, Eger, Hungary, July 19-27, 1997.
--. "Levitation Bearing for Flywheel and Vibration Isolation Devices Using HTS and Magnets," invited talk, The Flux, Quantum, and Mesoscopic Effects in Superconducting Materials and Devices, Santa Fe, NM, Aug. 4-8, 1997.

Contents
ISSO -- Institute for Space Systems Operations
1997-1998 Annual Report