Experimental study on on-orbit and launch environment vibration isolation performance of a vibration isolator using bellows and viscous fluid
Introduction
High precision payloads on board a satellite are exposed to vibration disturbances induced by the operation of various actuating components including reaction wheels, control moment gyroscope, thrusters, cryocoolers, solar array drives and other scanning components [1], [11], [14]. Although the vibration induced by the aforementioned sources is usually very small, even a micro-level vibration or jitter may cause serious degradation in the performance of high precision payloads [12], [15], [16]. In order to protect the high precision payloads from the vibration disturbances, vibration isolators are often applied in the vibration propagation path. Several vibration isolators utilizing viscoelastic materials have been developed for space application including advanced X-ray Astro-Physics Facility (AXAF [18]), James Webb Space Telescope (JWST [4]) and satellite ultraquiet isolation technology experiment (SUITE [1]). Viscous isolator called D-strut [7], [9], [20] has been developed and applied in various space programs including isolation of reaction wheel assembly in Hubble Space Telescope (HST [8]), vibration isolation and suppression system (VISS [5]) and miniature vibration isolation system (MVIS [17]).
Vibration isolators developed for on-orbit space application should be able to isolate micro-vibration when in orbit and also withstand the severe launch vibration environment. Because the space environment is very different from the environment where the ground tests are performed, on-orbit isolation performance could be significantly different from those evaluated through the ground tests. The material properties of commonly used viscoelastic materials and viscous fluids are heavily dependent on temperature, and vacuum environment can also cause changes in the material properties through outgassing. Although flight qualification tests have been performed on previously developed vibration isolators, the effect of space environment on the isolation performance is usually stated in changes of material properties and more complete information such as measured transmissibility has not been presented.
In this paper, the effects of thermal-vacuum environment on the isolation performance of vibration isolator using bellows and viscous fluid are examined experimentally in terms of the measured transmissibility. A test setup is prepared inside a thermal-vacuum chamber which allows the measurement of transmissibility of the isolator in thermal-vacuum environment. By using the test setup, problems caused by the elongation of the bellows due to the pressure difference in vacuum environment were observed. This problem was addressed by sealing the viscous fluid inside the bellows at a vacuum condition. The effect of temperature variation on the isolation performance was also observed and the test results show that the changes in isolation performance are confined to the narrow frequency region around the isolator's resonant frequency which is due to the isolator's three parameter configuration.
In addition to the on-orbit environment test, launch environment test is also conducted to check the structural safety of the isolator under the launch loads. Some modifications are made to the bellows to reduce the maximum induced stress and to increase the yield strength of the bellows. With these modifications, the isolator can undergo large deformation without being permanently deformed. The on-orbit and launch environment test results indicate that the developed isolator can provide effective isolation for the small amplitude vibration disturbances on-orbit and also survive the large amplitude vibration during the launch without any damage.
The organization of this paper is as follows. In Section 2, a description of the developed vibration isolator is given including modifications made to the isolator so that the isolator may withstand large deformation without being damaged and maintain its isolation performance in a vacuum environment. In Section 3, on-orbit vibration isolation performance is analyzed experimentally in a thermal-vacuum chamber and the effect of temperature variation on the isolation performance is discussed. Section 4 deals with launch environment test, and summary and concluding remarks are given in Section 5.
Section snippets
Development of vibration isolator
A vibration isolation platform for reducing on-orbit micro-vibration developed by the author's research group includes passive three parameter isolator [3], [6] made using bellows and viscous fluid (similar to D-strut) and the active components (voice coil actuators and force sensors). Because the vibration disturbances generated on-orbit are very small in amplitude, the isolator is designed to be sensitive to small motion. Bellows are used as a spring element and a fluid chamber which allows
On-orbit vibration isolation test
Thermal-vacuum test was conducted to check the isolation performance in a space environment. Although the temperature variation in the space is very large, vibration isolators are installed inside a satellite where the temperature is controlled to vary much less. The test temperature range was chosen to be (three cycles) and vibration isolation performances were measured at and at each cycle. The test was conducted at a component level (characteristics of the single-axis
Launch environment test
This section describes the launch environment test of the developed vibration isolator. The launch environment test (sine and random vibration tests and a shock test) is conducted to check whether the developed isolator can survive the harsh launch environment without any mechanical damage and to examine isolation effectiveness for the vibration induced during the launch. The test is performed on the multi-axis vibration isolation platform comprising isolators sealed in a vacuum environment.
Conclusion
Experimental study on on-orbit and launch environment vibration isolation performance of a vibration isolator using bellows and viscous fluid was conducted in this paper. Vibration isolation test in a vacuum environment revealed that isolation performance can change drastically from the one observed in ground condition due to the elongation of the bellows caused by pressure difference. To guarantee good on-orbit isolation performance, viscous fluid in the bellows isolator was sealed in a vacuum
Conflict of interest statement
None declared.
Acknowledgement
This research was supported by the NSL (National Space Lab) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (grant number 2009-0091934).
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