2축 김벌구동형 위성용 X-밴드 안테나의 미소진동 저감에 관한 연구

Abstract

Satellite on-board appendages that have mechanical moving parts such as a fly-wheel, a control moment gyro, a cyrocooler and a gimbal-type directional antenna produce an undesirable micro-vibration disturbances which can cause deleterious effect on the image quality of the high resolution observation satellite. Therefore, the micro-vibration attenuation is an important technology because the acceptable micro-vibration requirement is becoming much lower. To achieve a jitter-free silent platform, many researches have been conducted for enhancing image quality by using a micro-vibration isolation systems based on passive, active and semi-active techniques. Recently, the size of the image data transferred to ground station from satellite is also greatly becoming increased according to the recent tendency of satellite imagery markets which requires a much higher resolution image. Therefore, the high resolution observation satellites also require effective management of the handling and transmitting the massive high resolution image data from satellite to ground station by using an omni-directional or directional X-band antenna. The directional X-band antenna can be more effectively used for the massive image data transfer in a real time than the omni-directional antenna because it offers higher gain by pointing the antenna to specified direction without affection of attitude and orbital motion of the satellite. The pointing capability of the directional antenna can be increased by mounting it on multi-axis gimbal system and in general, a stepper motor combined with harmonic drive gear is used for the activation of the rotational movements of the multi-axis gimbal-type antenna. However, the stepper motor activation is one of the main sources to induce the micro-jitter disturbances, which can result in the image quality degradation of high resolution observation satellite. In order to mechanically attenuate the micro-vibration of the gimbal-type X-band antenna, mounting a whole antenna assembly on a soft mount isolation system might be one of the solutions, which is generally applied for the micro-vibration isolation systems for space applications. In this approach, the micro-vibration isolation can be achieved by frequency decoupling between the harmonic frequency of the antenna and the eigen-frequency of the antenna assembly supported by isolation system with low stiffness. However, the structural safety of the X-band antenna supported by the low stiffness soft mount isolation system cannot be guaranteed under the severe launch loads. This problem can be easily solved by applying a launch locking mechanism, but it might induce additional disadvantages such as the increase of total mass, system complexity and decrease in reliability. In addition, if the launch locking mechanism is not normally working on-orbit by unexpected problems, the micro-vibration isolation cannot be expected anymore. The ideal approach is to find out a more reliable and simply applicable solution, which can effectively attenuate the micro-vibration disturbances without employing the launch locking mechanism additionally to guarantee the structural safety of the soft mount whole antenna isolation system under launch environment. Therefore, Kozilek et al. proposed to implement a low-torsional-stiffness on the existing external spur gear wheel, which is mounted on the output shaft of the stepper motor actuator for the purpose of the micro-vibration isolation in the azimuth rotational direction of the antenna. The design provided a reliable technical solution that can be easily implemented, even on existing X-band antenna by just simple modification of the design. In addition, the effectiveness of the design was verified under the test condition when the antenna is activated by constant rotational velocity in azimuth direction. However, this is quietly different operating condition of the antenna based on the TPF (Tracking Parameter File) activation profile, which simultaneously drives the rotational movements of antenna in azimuth and elevation direction. In addition, the design method for implementing the low-torsional-stiffness on the gear wheel was not also introduced in the previous study. In this study, we investigate the possibility of using a low-torsional-stiffness isolator to enhance a micro-vibration attenuation capability for a stepper actuated X-band antenna mechanism. For this, we proposed to mount the low-torsional-stiffness isolator on output shaft of the stepper actuators for activating the rotational movements of antenna in azimuth and elevation directions. In case of micro-vibration attenuation in azimuth rotational direction of the antenna, the low-torsional-stiffness was directly implemented on the existing spur gear wheel based on the approach proposed by Kozilek et al. This approach is very effective in attenuating micro-vibration in the azimuth direction of the antenna. However, there are some limitations to implementing a low-torsional-stiffness spring-blade design on the elevation gear wheel, owing to the small, limited volume of the bevel gear that is directly mounted on the output shaft of the stepper motor actuator. Therefore, in this study, in order to achieve the micro-vibration attenuation in the elevation direction of the antenna, we proposed to implement the low-torsional isolator between the shaft of the stepper actuator and the bevel gear. These design approaches provide a reliable technical solution that can be easily implemented, even on existing hardware, without making major modifications to the conventional design of the antenna, by effectively attenuating the micro-vibration in both the azimuth and elevation direction of the X-band antenna. The low-torsional-stiffness isolators must guarantee both the micro-vibration attenuation capability and structural safety of the blades. Therefore, in this study, we performed the structure design of the isolators such that they are satisfied with the margin of safety rule when the required torque budgets for azimuth and elevation activation are applied to the low-torsional-stiffness isolators. The structure safety of the blades were also experimentally verified through the torque measurement test. To demonstrate the effectiveness of the low-torsional-stiffness isolator, we performed a micro-vibration measurement test of the X-band antenna using a floating-type micro-vibration measurement device proposed in this study. The micro-vibration measurement test was performed under constant rotational velocity condition. In addition, TPF activation profile, that drives the rotational movement of the antenna with various rotational velocity in both azimuth and elevation directions, was adopted to simulate an actual on-orbit operation of X-band antenna. The test results indicate that the low-torsional-stiffness isolators proposed in this study are effective in attenuating the micro-vibration induced by stepper actuators for activation of rotational movements of the antenna in azimuth and elevation directions.