Thomas Wilson. NASA Johnson Space Center, Houston, TX 77058. Absfruct - We employ the HTS-magnet interaction in the mechanical design of a vibration ...
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCHVITY, VOL. 9,NO. 2, JUNE 1999
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Vibration Isolation for Space Structures using HTS-Magnet Interaction Jang-Horng Yu, E. Postrekhin, Ki Bui Ma, and Wei-Kan Chu Texas Center for Superconductivity at the University of Houston, Houston, TX 77204
Thomas Wilson NASA Johnson Space Center, Houston, TX 77058
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We employ the HTS-magnet interaction in the mechanical design of a vibration isolator. One common element of space structures is the coupling between multiple substructures or mechanical parts. Often, such coupling needs to provide a transmission of force between the two systems while blocking out the propagation of the vibration energy from one system to the other. A solution to this is to establish a soft link between the two system. In this paper, we design a passive vibration isolation device employing the characteristics of the HTS-magnet interaction. The configuration of the vibration isolator consists of a ring magnet and a thin disk HTS where the €ITS is located in the middle of the magnet and is levitated. Experiments show that the natural frequency of the system is 4 Hz and the 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 that is not readily available in the space environment. The concept can also be used as an isolation platform and can combine with the active vibration isolation technology so as to attenuate the vibration of all frequencies.
I. INTRODUCTION High temperature superconductors (HTS) can be used in the levitation systems such as frictionless bearings, energy storage flywheels, satellite momentum wheels and vibration isolation devices. The applications of HTS are also suitable for space applications where the low temperature environment and vacuum condition is ideal for the YBCO material. The melt texture growth technology allows one to produce bulk high-Tc superconductors with a high critical current density and magnetic field. Due to this advance, the application of HTS is expected to grow. One common element of (space) structures is the coupling between multiple substructures or mechanical parts. Often, such coupling needs to provide a transmission of force between the two systems while blocking out the propagation of the vibration energy from one system to the other. HTSmagnet joint has a great potential to serve this purpose as a low-pass vibration filter due to the softness of the bearing. Understanding the dynamic response of the HTS-magnet joint is important in the aspects of mechanical design and vibration control. Nagaya has presented a numerical method Manuscript received May 28,1998. This work was supported in part by the Department of Energy and the State of Texas through the Texas Center for Superconductivity at the University of Houston and 'AFOSR # F49620-97-0101.
for controlling vertical vibrations of a levitated high-T, superconducting body [l]. The modeling and control of the vertical vibrations of HTS levitation system subjected to external disturbances have also been discussed by Nagaya et. al. [2]. Grosser et al. has investigated the damping of the translational oscillations of a permanent magnet levitated inside a superconducting parallel plate as a function of temperature and oscillation amplitude [3]. Here we report on a study of the horizontal vibrations of levitated superconductors above a ring permanent magnet. This system appears to hold promise as a passive vibration isolation device. 11. EXPERIMENT AND RFSULTS
In our investigation, we used samples that were prepared by melt-texturing with seeded directional solidification [4].Two samples were used. One is in the shape of a circular disk. The other is in the shape of a circular ring. The superconducting dlsk has a diameter of 40 mm and a thickness of 20 mm. The superconducting ring has the same outside diameter and thickness with a concentric hole of 16 mm diameter. The critical current density of the samples was on the order of 104A/cm2at 77K. as measured by four-contact impulse method on a small specimen cut from the bulk. A schematic diagram of the experimental setup is shown
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Fig.1. Schematic drawing of the experimentai setup.
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in Figure 1. A superconductor is mounted in a heat insulator cup held near the center of and at a certain fixed distance above a permanent Nd-Fe-B magnet in the shape of a ring 50 mm inner diameter, 70 mm outer diameter and 12 mm thick. This magnet supports a field of 0.5 T on either of the flat surfaces. It is attached to a table equipped with a shaker. The shaker is used to generate horizontal vibrations. When the superconductor is cooled below its critical temperature using liquid nitrogen and becomes superconducting, it is levitated . stably without any direct physical support, and remains so throughout subsequent measurements. (This is achieved by gluing the superconductor to the bottom of a Styrofoam cup, holding the cup above the magnet with a mechanical clamp and stand, putting liquid nitrogen into the cup until the superconductor cools sufficiently and remains immersed in liquid nitrogen, and then releasing the mechanical hold.) Horizontal vibrations from the shaker are rigidly coupled to the ring magnet and a certain fraction of this is further transmitted to the levitated superconductor via the supporting magnetic field. To measure this fraction, which is the transmissibility across this HTS-magnet joint, an accelerometer is fixed to the heat insulator cup and another on the magnet. The signals from these accelerometers representing the accelerations of the HTS and magnet ring respectively sent to a dynamic signal analyzer for analysis. The transmissibility is obtained as the ratio of the amplitude of the acceleration of the HTS to that of the magnet ring. 111. RESULTSAND DISCUSSION The horizontal vibration transmissibility for the superconducting disk (square symbols) and ring (circle symbols) are shown in Figure 2. The superconducting ring was levitated at a distance of 2 mm above the ring magnet, and the disk, 3 mm. For comparison purposes, the transmissibility obtained when the superconductor is not levitated, but allowed to settle on and not fixed to the ring magnet and shaker is also included in the same figure using triangle symbols. (This position of the superconductor is attained by letting the temperature of the superconductor go up above its critical temperature so that the superconductor reverts to its normal non-superconducting state. Then the superconductor falls gently and comes to rest either on top of the disk magnet, or through the hole of the ring magnet onto an aluminum plate which serves as holder to the magnets and attaches them to the shaker.)
As seen in Figure 2, a few resonance peaks are found for the horizontal vibration transmissibility. There is a sharp resonance at the natural frequency of about 4 hz for both the superconducting ring and disk. The presence of multiple peaks is the result of the existence of multiple modes of vibration that can be excited by horizontal oscillations. For instance, the lowest mode is horizontal translation of the levitated superconductor. Higher modes mix in rotational motion around the vertical axis. The transmissibilitywhen the superconductor is resting directly on the ring magnet and
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Frequency f (Hz) Fig. 2. Transmissibilityas a function of frequency of external disturbance.
shaker is higher then that that with the superconductor levitated. This means that the combination of the levitated superconductor above the ring magnet serves as a vibration isolation device, preventing vibrations from the shaker from reaching instrumentation that might be mounted on the levitated superconductor. It is well known that the vibration isolation performance depends on the natural frequency of the link between two mechanical parts. Vibrations are effectively isolated for frequencies that are larger than 62 times the natural frequency of the link. Therefore, if a "weak" link is established between the two systems, indeed, a large range of vibration frequencies can be successfully isolated. Suitable configuration of the HTS-magnet interaction can provide the equivalent of a weak spring. Our experiments show that
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T i m (s) Fig. 3. Response of accelerometeras a function of time at free damping vibrations.
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frequencies above 20 hz are successfully isolated for more than 30 dB. Such a passive device in space applications is superior to similar active devices that often require bulky control circuit boxes and consume considerable energy that is not readily available in the space environment. The concept can also be used as an isolation platform that can also incorporate active vibration isolation technology so as to attenuate vibrations of all frequencies. For a more detailed characterization of the performance of the vibration isolation device, the damping frequency is also important. This damping frequency, k, for first mode can be obtained from the width of the highest resonance peak divided by 42 and is practically the same for the disk or the ring. For our setup, this frequency is 4.4 Hz. The damping frequency was also determined for the levitated superconductor from kee but damped vibrations following an impulse. Using a pulse function generator to generate impulses of 100 ms durations, we observed the impulse response of the levitated superconductors. A typical response of the accelerometer attached to the superconductor is shown in Figure 3. Assume that one horizontal vibration mode is more excited than any other mode, the system exhibits damped vibrations changing in time as x = Aoe-”sin(W + 90) where A. and q, are constants. A = is the instantaneous amplitude of the damped oscillations from which k may be estimated. From the actual experimental data of the damped vibrations, we estimate k to be 4.3 Hi for the disk and 4.1 Hz for the ring. The damping coefficient is given by c = 2mk. In our case, the mass m of the disk and ring is 0.136 kg and 0.101 kg respectively and c for the disk and the ring comes to 1.16 Ns/m and 0.83 Ns/m respectively. HTS can be applied to many mechanical designs to change the dynamic response of a structure. A vibration isolation device such as that described can still be used to transmit forces at low frequencies since the transmission ratio below 1.414 w, is always greater than 1. This will allow the space structure elements to interact with one another especially when in rigid body modes at slowly varying speeds (e.g., move from one place to another). However, such links will be excellent for isolation of mechanical vibrations from machinery at Erequencies of the order of tens or hundreds of hertz. In other words, these links can be used as a dual-task component; i.e., to transmit force at ultra-low frequencies and to isolate vibration at the higher frequencies.
IV. CONCLUSION We have investigated the horizontal vibrations of a system consisting of a superconductor (disk or ring) levitated over a ring magnet. The resonant fiequency and the damping coefficient of such a system was found based on experimental transmissibility data. A resonmt frequency of 4 hz was
achieved for horizontal vibrations that is comparable with other passive vibration isolation systems which are much more massive and bulky. The damping coefficient of this system is acceptable, but for real applications or asa supplement to active vibration isolation, additional damping may be required. However, it is possible to modify the present design to achieve higher damping ratios. Conventional bearings or joints between two mechanical structures or elements cannot function normally in outer space where the temperature is extremely low and the condition resembles that of a vacuum. Active magnetic bearings or levitation platforms may provide a suitable alternative but they require constant energy input that is not readily available. Therefore, passive HTS-magnetic bearingdjoints can be used at a fair advantage for space applications. As the critical temperature and the critical current of HTS become higher, we can expect that the use of HTS in space will be more widespread. ACKNOWLEDGEMENT
Tbe authors would like to thank Professor K. Salama and Dr. M. Mironova for supplying the samples in our studies and Dr. Quark Chen for useful discussions. This work was supported in part by a joint postdoctoral fellowship from the Institute for Space System Operations at the University of Houston and Johnson Space Center of the National Aeronautics and Space Administration, by the Department of Energy and the State of Texas through the Texas Center for Superconductivity at the University of Houston and the United States Air Force under AFOSR grant # F49620-97-0101. REFERENCES [11 Kosuke Nagaya, “Analysis of a high-Tc superconducting
levitation system with vibration isolation control,” ZEEE Trans. Magn. vol32, pp.445-452, 1996. [2] K. Nagaya, M. Tsukagoshi, and Y. Kosugi, “Vibration control for a high-Tc superconductingnon-linear levitation system,” Journal ofsound and Vibration vol 208, pp. 299-311, 1997. [3] R. Grosser, J. Jager, J. Betz, and W. Schoepe, “Damping of the oscillations of a permanent magnet levitating between high-Tc superconductors, ” Applied Physics Letters vol67, pp. 2400-2402, 1995. [4] D. F. Lee,C. S. Partsinevelos, R. G. Presswood Jr., and K. Salama, “Melt texturing of preferentially aligned YBa-Cu-O superconductor by a seeded directional solidification method,” J.AppZ.Phys. vol76, pp. 603-605, 1994.
