Wei-Kan Chu, Ph.D., Professor, UH
Ki Bui Ma, Ph.D., Research Associate Professor, UH
Thomas Wilson, Ph.D., JSC
Jang-Horng Yu, Ph.D., Post-Doctoral Fellow, UH
RESEARCHERS ARE INVESTIGATING the benefits of
using high temperature superconductors (HTS) and magnet interaction in the mechanical
design of vibration isolated platforms and structural damping devices for space
applications. Experiments are being conducted to determine the dynamic characteristics of
HTS-magnet interactions for the purpose of optimizing the performance of the HTS-magnet
mechanism.
One common element of the space structure is the coupling (or joint) between two mechanical parts, e.g., a space telescope and a sun shield. Such mechanism requires a force transmission between two systems and a mechanism for blocking the propagation of vibration energy. Such design requires non-contact forces so as to damp out the vibration energy associated with very low frequency (below 1 Hz) since vibrations are difficult to isolate when two bodies are in physical connection. Conventional bearings are normally contact joints that often require lubricants. In outer space, however, the temperature is extremely low, and the condition resembles that of a vacuum. Therefore, lubricants will either freeze or evaporate and they cannot function properly in such harsh conditions. There is a need to invent new materials that provide non-contact forces with the capability to transmit the desired forces.
Dr. Wei-Kan Chu, UH (l.), Dr. Thomas Wilson, JSC (l. center), Dr. Ki Bui Ma, UH (r. center), and Dr. Jang-Horng Yu, Ph.D., Post-Doctoral Fellow, UH-JSC (r.), test high temperature superconducting magnets.
Reducing the energy consumption of these devices on board the space station is crucial since the energy supply is limited. To this purpose, passive devices are superior to active devices. Passive devices, however, such as the shock absorber of the automobile operate with fluids and the principles of fluid flow, with viscosity as the dissipation mechanism. These devices cannot function properly in the space environment for lack of a suitable fluid that would have the right viscosity at the typically low ambient temperatures (between 30 and 100 K) involved. Under such conditions, one current technique is eddy current damping.
The eddy current damper consists of a permanent magnet and a good electrical conductor in close proximity, but not in direct contact, so that the magnet moves relative to the conductor in the presence of vibrations. The ohmic losses accompanying the eddy current induced in the conductor by this motion dissipate the mechanical energy of the vibrations. In a similar manner, and with even better results, a permanent magnet with HTS can be used to damp out vibrations although the dissipation mechanism here is not resistive loss, but magnetic hysteresis, originating from flux pinning forces in the superconductor.
Since HTS works well in low temperatures and an HTS bearing is a non-contact joint that does not require lubricants, HTS material is naturally selected to design a vibration isolation mount that can operate in the outer space condition.
Objectives
The central concept behind the development of a vibration free platform for
experimentation has the hardware involved couple mechanically with the rest of the
spacecraft only through HTS-magnet vibration dampers at appropriate sites. Even at these
sites, the HTS which is attached to the spacecraft and the magnet attached to the
experimental hardware are still not in direct contact with each other; the experimental
hardware is locked in position by the magnetic flux that threads through both components
and is prevented from moving by the pinning force inside the HTS. The same occurs if the
arrangement is reversed with the magnet on the spacecraft and the HTS on the experimental
hardware. Furthermore, the distance between the HTS and the magnet can be adjusted in
order to achieve the specific requirements for vibration isolation. For example, if the
low frequency vibration is to be isolated, researchers shall make the "spring"
between the HTS and the magnet soft enough so that the natural frequency of the system is
small. However, if the vibration damping of the system is the main concern, an optimal
distance can be chosen to yield maximum energy consumption. With this vibration free
platform in space, vibrations from the spacecraft, as well as self-induced vibrations
within the experimental hardware on the platform itself, would see these dampers as sinks
of the vibration energy. Because there is no direct contact, wear and tear would also be
absent.
Understanding the dynamic characteristics of HTS-magnet interaction is not only an important issue of scientific interest in the innovation of space technology but of great economic potential concerning HTS applications in industry. Although superconducting levitation with HTS has been used in the design of bearings for flywheels, the nonlinear dynamic characteristics of HTS-magnet interaction have not been thoroughly understood. This information is especially crucial in designing a vibration isolation platform or damping device. Providing a theoretical model and experimental data will enhance the performance as well as improve the future design of HTS-magnet devices. Our objective is therefore to provide the fundamental understanding of HTS-magnet interaction such as dynamics stiffness and damping coefficient through the proposed theoretical model and experimental verification.
Methodology
Here, we briefly summarize the ideas behind constructing HTS-magnet devices. Experimental
results described in this report have been reported previously in the references cited
below.
Vibration Damping
In vibration damping studies, we compare an HTS-magnet damper to an eddy-current damper
(copper-magnet). We have installed an HTS-magnet vibration damper on a cantilever beam as
shown schematically in Fig. 1. The vibration damper
consists of a rectangular slab of HTS in close proximity to a magnet slab. The magnet slab
is, in turn, comprised of slices of permanent magnet with opposite polarity adjacent to
each other, as illustrated in Fig. 2.
UH investigators have studied the responses of the cantilever beam to an impulse and a continuous sinusoidal force applied near the free end of the beam, and compared findings with corresponding responses when the HTS is replaced by a copper slab of the same size. Figure 3(a) shows the impulse response of the cantilever beam with the HTS-magnet vibration damper, whereas Fig. 3(b) displays the response with a copper-magnet vibration damper. Clearly the vibrations are damped out much more quickly with the HTS-magnet device, compared to the copper-magnet device.[3]
As for continuous excitation, Fig. 4 shows the comparison between the force response curves of the cantilever beam at different frequencies due to the HTS-magnet damper and the copper-magnet damper. With the copper-magnet damper, there is a sharp resonance peak, very much like the resonance response of the free beam. With the HTS-magnet damper, there is a low broad peak, indicating that the resonant vibration of the beam without the damper is very much suppressed by the HTS-magnet damper. However, the position of this peak is shifted to higher frequencies and, when combined with its large width, renders the effect of the HTS-magnet damper comparable with that of the copper-magnet damper for frequencies above the natural frequency of the cantilever beam. Apart from this slight complication, the HTS-magnet damper exhibits a higher overall damping capability, particularly at low frequencies.
Vibration Isolation
It is well-known that vibration isolation can only be achieved when the forcing
frequencies are at least √2 times greater than the natural frequency of the
system. In space, for example, vibrations in the sub-Hertz domain are known due to the
motion of flexible space structures and the work of astronauts. In this case, consider an
HTS-magnet coupler to be a link between two systems. If we can lower the natural frequency
between the HTS and the magnet, i.e., to provide a near zero stiffness spring
between the HTS and the magnet, this result will imply that the natural frequency of the
coupler between two systems is also near zero. Therefore, the sub-Hertz vibration can be
effectively isolated one from another.
The force-distance curve of the HTS-magnet interaction surely gives a promising feature of this idea. Figure 5 shows the force-displacement curve of an HTS-magnet coupler. Note that when the distance between the HTS and the magnet is large enough (greater than 0.6 mm), we see a flat plateau with the equivalent spring constant close to zero and so will be the natural frequency of the HTS-magnet coupler. Due to the microgravity condition in outer space, although the restoring force is small, it shall be able to isolate the vibration energy effectively and also provide a force transmission between two systems.
Project Status
In the previous Annual Report (1995-96), we proposed different mechanical designs of
HTS-magnet non-contact joints for space structures. We have completed the experimental
design to study the dynamic characteristics of HTS-magnet interactions by building an
HTS-magnet joint whereby the HTS is attached to a vibration shaker and the magnet hung by
a very soft elastic spring (Fig. 6). The signal is then
fed through a spectrum analyzer to study the dynamic response (Fig. 7). The natural frequency of the HTS-magnet system is
measured to be 6 Hz. Vibrations of frequencies greater than 10 Hz are thus successfully
isolated. Without complete simulation of microgravity condition, it is difficult to extend
the experiment to frequencies lower than 1 Hz. Furthermore, in UH attempts to attenuate
the acceleration of the low frequency vibrations down into the range of micro g's,
output signal from our accelerometer falls to the background noise level, rendering data
acquired through the dynamic signal analyzer unreliable. To remedy this situation, we have
built a reflective optical sensor to measure displacements. This approach yields a much
better signal-to-noise ratio. We are in the progress of acquiring data by this alternative
method.
Magnetic hysteresis in HTSs allows continuum equilibrium states under the mutual forces between a permanent magnet and a HTS. Furthermore, flux creeping in the HTS means that either the magnet or the HTS can creep within this continuum of equilibrium states. As an example, when we levitate a piece of magnet above a cooled superconductor, the gap size changes slowly as time elapses. To describe this relaxation phenomenon, we have proposed a history-type material model (identified as "material with memory") to describe such behavior.[1,2] We have also designed an experiment to measure the stiffness and damping coefficient as a function of the gap size by attaching a magnet to a guided linear bearing. This signal will be measured again by the reflective optical sensor that we constructed. The measured data will be reported in the near future.
Apparently, the designs of permanent magnets and HTS in close proximity are good candidates for incorporation into the construction of microgravity experimental hardware-carrier and payloads-as passive devices that limit self-induced disturbances and isolate them from spacecraft disturbances. By suppressing disturbances as much as possible, we hope to improve the conditions under which certain experiments, can proceed, such as crystal growth, and further refine the performance of other advanced instrumentation, most notably, position sensitive sensors, such as communications receptors, infrared and optical telescopes, and scanning force microscopes in terms of either their spatial resolution or scan rate. This refinement, however, can enhance the capabilities of instrumentation to such an extent that potential new directions of applications can take hold. Eventually, our intention is to develop an entire system based on HTS and magnets to control and manipulate the positioning of such instrumentation with great speed and minimal tolerances. Our near term goal is to be able to provide an environment for experimentation that approaches the ideal of being free from extraneous vibrations, using HTS-magnet isolators/dampers.
References
[1]M. A. Lamb, K. B. Ma, C. K. McMichael, R. L. Meng, P. H. Hor, R. Weinstein, I. Chen,
and W. K. Chu. "Characterization of Non-Contact Vibration Absorbers Using YBa2Cu3O7,"
presentation, 1992 TCSUH Workshop on HTS Materials, Bulk Processing and Bulk Applications,
Houston, TX, Feb. 1992.
[2]K. B. Ma, C. K. McMichael, M. A. Lamb, and W. K. Chu. "Application of High
Temperature Superconductors on Levitation Bearings, Torque Transmissions and Vibration
Dampers," presentation, 1992 Applied Superconductivity Conf., Chicago, IL, Aug. 1992.
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