As recent satellite imagery markets require much higher resolution images, the size of satellite image data transmitted to ground stations is generally increasing. Therefore, high-resolution observation satellites require an effective management strategy to transmit huge and high-resolution image data to the desired ground station through omnidirectional or directional X-band antennas. However, since the directional X-band antenna can directly point the antenna in a specific direction without being influenced by the attitude of the satellite or the movement on the orbit, it can transmit the large-capacity image data more effectively in the real-time mission than the omnidirectional antenna. The effective pointing ability of the directional gimbal-type antenna can be improved by attaching it to a multi-axis gimbals system operated by a stepper motor. Such a X-band antenna with multi-axis gimbal system is installed outside the satellite and experiences a severe launch vibration environment until it enters the target orbit. The above-mentioned severe launch vibration environment include a dynamic disturbance such as a quasi-static load, random vibration and shock. The various launch loads affect the two rotational drives parts provided to direct the antennas of the ground station irrespective of the orbital motion and attitude of the satellite in the operation of the 2-axis gimbal-type X-band antenna. Therefore, a launch locking system is applied to enable successful survival from the various launch loads in order to perform a successful mission of the X-band antenna. In addition, a stepper motor is used to operate the two rotation driving parts and a gear module having a constant gear ratio is used to transmit the power of the motor to the two rotation driving units. However, the micro-jitter disturbances caused by imperfect intermeshed teeth of the harmonic drive gear configuration and dynamic imbalance of the whole antenna system during the stepper motor activation are one of the serious micro-jitter sources to degrade the image quality of the high resolution observation satellite. Therefore, Kozilek et al. proposed a titanium gear with a blade shape for the purpose about attenuated micro-vibration generated during azimuth rotational driving of a 2-axis gimbal-type X-band antenna. Because of the low rotational stiffness due to the blade shape, the desired micro vibration reduction performance is ensured by simple shape change of the gear without changing the complex design of the antenna hardware through frequency separation. However, when the above gear is applied, unexpecting over driving torque is applied to the titanium blade gear due to the actual antenna assembly process or alignment imbalance of gear axis caused by the severe launch vibration loads or orbit-thermal environment. At this time, plastic deformation occurs in the titanium blade, and there is a problem that it is impossible to transmit accurate power for driving the antenna from the motor shaft. In terms of the structural safety, above mentioned problem can be easily solved by increasing the stiffness of the blade shape to guarantee the structural safety of the titanium gear under the condition that a much higher driving torque is induced. However, this approach might result in degrading the micro-jitter attenuation capability because the function of attenuating the micro-jitter depends on the torsional stiffness of the gear. In other words, the micro-jitter attenuation capability cannot be expected anymore with the highly stiffened blade gear design. Therefore, We focused on pseudoelasticity behavior of Shape Memory Alloy (SMA) to overcome the problems of the titanium blade gear proposed by Kozilek et al.. pseudoelasticity behavior is a characteristic that significant deformations under excessive loading and recover its original shape when the load is removed, without undergoing plastic deformation, which is caused by a stress induced phase transformation. Therefore, In order to attenuate the micro-vibration generated at the azimuth stage of the 2-axis gimbal-type antenna, We proposed gears with SMA mesh washer. This application makes it possible to achieve a lower torsional stiffness and higher damping in the torsional direction of the gear. The lower torsional stiffness and higher damping are main contributors to the micro-jitter isolation capability of the gear when the stepper motor is actuated. In addition, the pseudoelasticity of the SMA mesh washer guaranteed the structural safety of the gear itself even under unexpected circumstances such as over-driving torque induced into the gear.
In this study, we describe the structural design of a two-axis gimbal-type X-band antenna and analyze the dynamic response characteristic of the antenna under different constraint conditions depending on whether or not the launch lock device is constrained under launch and on-orbit environment. In addition, the quasi-static analysis was performed to confirm the structural safety of antenna structure and bolt I/Fs between antenna base and satellite. The suitable range of constraint force on launch lock device was also determined to ensure the structural safety and mechanical gapping of ball & socket Interfaces, which provides multi-constraints on the azimuth and elevation stage of antenna. and, This paper describes the introduction and structural design of the proposed SMA mesh washer gears for micro-vibration isolation that occurs when the mission of the X-band antenna is performed in orbit. In order to verify the design validity of the gears, basic characteristics tests for gears, static load tests under various temperature conditions, and accelerated life tests were carried out. The effectiveness of the micro-jitter attenuation capability of the SMA mesh washer gear was demonstrated by the miro-jitter measurement tests by using a 2-axis gimbal-type stepper actuated x-band antenna.