Geeyong Park

Development of multi-degree-of-freedom microvibration emulator for efficient jitter test of spacecraft

A number of components and payload systems installed in satellites are found to be exposed to various disturbance sources, such as the reaction wheel assembly, the control moment gyro, coolers, and others. Low damping in space causes continuous microvibration that degrades the performance of various payload systems; this is customarily called jitter problem in spacecraft community. Various isolation systems have been developed to improve the performance of the optical payloads under the microvibration environments. During the spacecraft development process, an experimental validation of jitter effects (either controlled or not) on the performance of optical payloads using all the flight model components is not often conducted due to the scheduling issue or the product assurance activities. The disturbance characteristics of a prototype or an engineering model of the disturbance source such as reaction wheel assembly are significantly different from their flight model counterparts. Therefore, in order to facilitate jitter test as well as the performance evaluations of an isolator during the satellite development process, this article proposes a microvibration emulator that generates the real disturbance spectrums of flight models. The development procedure for the reaction wheel assembly emulator is described. Prototype single-axis and three-axis microvibration emulators are demonstrated, and the performances are evaluated by means of disturbance characteristics similar to those of the reaction wheel assembly.

Development of vibration isolation platform for low amplitude vibration

The performance of high precision payloads on board a satellite is extremely sensitive to vibration. Although vibration environment of a satellite on orbit is very gentle compared to the launch environment, even a low amplitude vibration disturbances generated by reaction wheel assembly, cryocoolers, etc may cause serious problems in performing tasks such as capturing high resolution images. The most commonly taken approach to protect sensitive payloads from performance degrading vibration is application of vibration isolator. In this paper, development of vibration isolation platform for low amplitude vibration is discussed. Firstly, single axis vibration isolator is developed by adapting three parameter model using bellows and viscous fluid. The isolation performance of the developed single axis isolator is evaluated by measuring force transmissibility. The measured transmissibility shows that both the low Q-factor (about 2) and the high roll-off rate (about -40 dB/dec) are achieved with the developed isolator. Then, six single axis isolators are combined to form Stewart platform in cubic configuration to provide multi-axis vibration isolation. The isolation performance of the developed multi-axis isolator is evaluated using a simple prototype reaction wheel model in which wheel imbalance is the major source of vibration. The transmitted force without vibration isolator is measured and compared with the transmitted force with vibration isolator. More than 20 dB reduction of the X and Y direction (radial direction of flywheel) disturbance is observed for rotating wheel speed of 100 Hz and higher.

Development of Multi-DOF Active Microvibration Emulator

Recently, some components and payload systems installed in satellites are exposed to various disturbance sources, such as the reaction wheel assembly, the control moment gyro, coolers, and others. Because there is low damping in space, the continuous microvibration causes the degradation of the performance of various payload systems. Therefore, the development of a practical isolation system that shields against microvibration are very important and the author is on the way to developing the microvibration isolation system for the improvement on the performance of the optical payload. In order to develop appropriate microvibration isolation device for a specific payload, it is necessary to understand vibration characteristics of the main disturbance sources; modeling and analysis of disturbance sources including reaction wheel assembly and control moment gyros have been studied by many researchers. However, there are practical difficulties to obtain and perform an experiment with real flight model (FM) reaction wheel assembly and control moment gyros because of expensive price and security reasons. Generally, the disturbance characteristics of a prototype of the reaction wheel assembly or control moment gyros are significantly different from those of FM ones even when the reaction wheel type, size and wheel speed are the same.Therefore, in order to facilitate the microvibration isolation experiment during the satellite development process, this paper proposes a microvibration emulator that could generate the real disturbance spectrums of FMs. Note that the disturbance profiles are quite complex, consisting of several higher harmonics, and also changing for varying operational wheel speeds. The disturbance characteristics of FMs are typically measured in advance. First an analytical model for the RWA is presented and the development procedure for the emulator is also described. The performance of the first prototype emulator is demonstrated.Copyright