VELOCITY OPTIMIZATION METHOD OF X-BAND ANTTENA FOR JTTER ATTENUATION

Transcription

1 The 21 st International Congress on Sound and Vibration July, 214, Beijing/China VELOCITY IMIZATION METHOD OF X-BAND ANTTENA FOR JTTER ATTENUATION Dae-Kwan Kim, Hong-Taek Choi Satellite Control System Team, Korea Aerospace Research Institute, Daejeon, Republic of Korea A 2-axis gimbal system is commonly adopted as a pointing mechanism to satellite antenna apparatuses. The gimbal system generally consists of azimuth and elevation stages which can be rotated simultaneously and separately about azimuth and elevation axes by stepper motors. As a result of the dynamic coupled motion between the stages, the moment of inertia and structural modes of the antenna assembly have time-variant characteristics, and thus the antenna assembly generates nonlinear and unpredictable dynamic disturbances. Therefore, the gimbal system is regarded as one of significant vibration sources in the spacecraft such as a reaction wheel assembly and a cryogenic cooler that affect the stability and image jitter. In this study, a velocity optimization method is proposed for an X-band antenna in order to attenuate the jitter response. The optimized target profile of the antenna is generated through the optimization process with some designated angular velocities, and then the applicability of the optimization method is evaluated through a micro-vibration test of the X-band antenna. The test result shows that the jitter response can be attenuated by about % on average using the velocity optimization method. 1. Introduction The line of sight (LOS) jitter on an imaging payload of a satellite is defined as the time varying motion of the image on the detector plane during image acquisition, which may be caused by internal and external disturbances acting on the optical system. 1 In general, the jitter is also referred to as a high frequency random motion of the satellite structure that affects the image, and it is regarded as an important factor to determine the quality of the image. This means that, for successful development of a satellite, it should be performed as a principal task to strictly expect the jitter performance and verify the requirement compliance through analytical or experimental approaches. In case of dissatisfaction with the desired jitter requirement, a particular method or plan should be prepared to mitigate the jitter. The internal jitter contributors of satellites are commonly the mechanical components which have moving parts; such as a reaction wheel assembly (RWA), a control moment gyroscope (CMG), a rotating gimbal system, a solar array driving mechanism, and a cryogenic cooler. 2,3 The gimbal system which is shown in Fig. 1 is used as a pointing mechanism of an X-band antenna. 4 In the satellite configuration, two 2-axis gimbal systems are adopted so as to rotate the primal and secondary antenna mechanisms (XAM-A and XAM-B) toward a desired ground target in real time. As shown in Fig. 2, each gimbal system consists of the azimuth and elevation stages, and these are rotated by two stepper motors about the azimuth and elevation axes, respectively. As a result of the dynamic ICSV21, Beijing, China, July 214 1

2 21st International Congress on Sound and Vibration (ICSV21), Beijing, China, July 214 Figure 1. LOS jitter phenomena due to X-band antenna operation. Figure 2. Configuration of X-band antenna driving system. coupled motion between the azimuth and elevation stages, the moment of inertia and structural modes of the antenna system completely change depending on the antenna pointing direction. In addition, the stepper motors generate nonlinear and broadband frequency disturbances resulting from the discontinuous rotation motion of their rotors and the harmonic excitation due to the gear box.,6 Consequently, the LOS jitter of the optical payload, which is caused by the X-band antenna mechanism (XAM), is difficult to precisely predict; thus, unexpected excessive jitter response can be generated in some cases. 7 For this reason, a jitter attenuation method applicable to the gimbal system should be developed in order to ensure the jitter performance. In the present study, a velocity optimization method applicable to a real flight XAM is proposed in order to attenuate the LOS jitter. The original angular profile of the antenna is modified through the optimization process, and then applied to a micro-vibration test on the X-band antenna assembly. Finally, the performance of the optimization method is invested through evaluating the micro-vibration response. 2. Velocity optimization of X-band antenna Axis gimbal system The 2-axis gimbal system has two stepper motors for rotation on the azimuth and elevation axes, respectively. The motor is a two-phase stepper type, and it has a harmonic gear of :1 gear ratio and 8 micro-stepping mode. There is no rotation limit in the azimuth stage, but the rotation limit from 1 to 14 in the elevation stage. The antenna horn located at the center of the elevation rotation axis has been designed to have a beam coverage of ±1 for an adequate ground contact condition. In the XAM used in this study, a vibration isolator which is a spring blade type is adopted at the azimuth spur gear box, so that the micro-vibration caused by the azimuth rotation can be reduced by -3dB. The elevation stage, however, does not have such an isolation mechanism due to a geometric limitation in its gear box; thus, the micro-vibration generated by the elevation motion can induce a significant jitter on the optical payload. Therefore, a jitter attenuation method, which can be used for the X-band antenna operation, is necessary in order to ensure an adequate performance of the jitter in a normal mode condition during on-orbit flight. ICSV21, Beijing, China, July 214 2

3 21st International Congress on Sound and Vibration (ICSV21), Beijing, China, July Velocity optimization method As mentioned in the preceding section, the antenna has been designed to achieve the beam width of 1. This means that the antenna motion profile can be changeable in the beam width without losing the ground contact. In this study, as the jitter attenuation method for the XAM, a velocity optimization method is proposed. The purpose of the optimization method is to generate an antenna motion profile using only selected angular velocities, so that the micro-vibration caused by the XAM can be prevented from exciting structural modes of the optical system. The velocity optimization process consists of steps as follows: 1) Generation of the motion profile using the data: Opt_rates, inputs, and Evaluate profile 2) Selection of the optimal segments: Evaluate segment values 3) Velocity optimization in each segment: Velocity optimization process (function ) 4) Round off at corners to minimize acceleration: Curve fitting at corner ) Generation of the data using the optimized profile: Evaluate and output The is the optimized target profile of the original target profile () which is established using the geometrical information between the attitude of the satellite on the orbit and the target point on the ground. Figure 3. Flow chart of antenna optimization. ICSV21, Beijing, China, July 214 3

4 21st International Congress on Sound and Vibration (ICSV21), Beijing, China, July Example of optimized In this study, the optimal velocities are selected as.8 deg/sec, 1.6 deg/sec, and 3.2 deg/sec for the elevation motion. The optimized profile can be produced using any combination of the designated velocities with the constraint that the center of the beam should be located in the beam width. For instance, the motion profile of a sample mission is shown in Fig. 4, where the target profile is optimized about the elevation motion. It shows that the elevation angle is optimized within the beam coverage using only the velocity of 1.6 deg/sec. 4 Sample Mission Profile : Azimuth 1 Sample Mission Profile : Elevation Azimuth angle (deg) Azimuth velocity (deg/sec) Elevation angle (deg) Elevation velocity (deg/sec) Figure 4. Angular position and velocity of the and for Sample Mission profile: azimuth (left) and elevation (right). 3. Micro-vibration test and verification 3.1 Micro-vibration test In order to evaluate the velocity optimization method, the micro-vibration test was performed on the XAM. Figure shows the configuration of the micro-vibration measurement system. Using a solid bracket, the XAM was mounted rigidly on a marble block supported by an air-floating isola- X-Band Antenna (APM) KISTLER Dynamic Force Platform Power input XAM Command input XDM EGSE KARI bracket Marble Isolators FM APD X Reference frame Y Seismic floor Force platform output Z Mounting structure Vertical offset Charge Amp. Anti-aliasing Filter DSP Board (Wheel speed control) KISTLER platform Figure. Micro-vibration test configuration of X- band antenna mechanism (XAM). Figure 6. X-band antenna mechanism mounted on the KISTLER plate. ICSV21, Beijing, China, July 214 4

5 21st International Congress on Sound and Vibration (ICSV21), Beijing, China, July 214 tion system installed on a seismic isolation floor. In this micro-vibration test, a quartz type dynamometer (Kistler/92B) was used to simultaneously measure the six components of the disturbances (i.e. three forces and three moments). Figure 6 shows the real configuration of the XAM installed on the Kistler table. For the test, the XAM was operated by the and profiles of the sample mission motion. All test data were measured using a data acquisition system (dsapce/ds113) at sampling rate of 248 Hz through an anti-aliasing filter (KROHB-HITE/3384). 3.2 Micro-vibration disturbances Figure 7 shows the disturbance data measured from the micro-vibration test, the disturbance forces (Fx, Fy, Fz) and the disturbance moments (Mx, My, Mz ), respectively. The coordinate system of the XAM is depicted in Fig. 6. The disturbance result clearly demonstrates that the axial moment (Mz) is relatively small in comparison with the other disturbances, and the main disturbances are produced in lateral forces (Fx, Fy) and moments (Mx, My). In addition, the most disturbances caused by the profile are generated as a result of the elevation stage motion. The frequency responses of the measured disturbance data are obtained by the use of a Fourier transformation method. The waterfall plots of the radial force (Fx) are shown in Fig. 8, which clearly exhibit that the most peaks of the frequency response are produced by the elevation motion. 1 Sample Mission - Disturbances Sample Mission - Disturbances Fx (N) Mx (Nm) Fy (N) My (Nm) -1-1 Fz (N) Mz (Nm) Figure 7. Micro-vibration disturbances measured using the and for Sample Mission profile: forces (left) and moments (right). Figure 8. Waterfall plots of the radial disturbance force (F x ): (left) vs. (right). ICSV21, Beijing, China, July 214

6 21st International Congress on Sound and Vibration (ICSV21), Beijing, China, July Micro-vibration reduction performance The disturbance reduction performance between the and profiles is evaluated in terms of the peak and standard deviation (STD) of the time response, and the peak and average of the root square sum of the frequency response. The values of the micro-vibration disturbance reduction ratio (%) are listed in Table 1. The reduction ratio is defined as follows: Reduction Ratio 1 (1) where and are the evaluated values of the micro-vibration disturbances measured to the and profiles, respectively. The result shows that, using the velocity optimization method, the micro-vibration caused by the XAM motion can be reduced by about % on average as the 1σ value. From the overall results, it is expected that the jitter response of the X-band antenna assembly can be effectively attenuated by the velocity optimization method proposed in this study. Table 1. Micro-vibration disturbance reduction ratio for the Sample Mission profile. Micro-vibration Reduction Ratio (%) Peak of disturbance STD of disturbance Peak of frequency RSS (~4Hz) Average of frequency RSS (~4Hz) Fx Fy Fz Force Mean Mx My Moment Mean Conclusions The velocity optimization method for the X-band antenna assembly is proposed in order to attenuate the jitter response of a satellite. The profile optimization method has a constraint that the angular velocity must consist of only some designated velocities. The optimization method was adopted to the XAM in order to verify its performance. The target profile of the XAM was optimized about the elevation motion using the optimization method. The micro-vibration test was performed on the XAM with the and profiles. The disturbance characteristics were investigated through analysing the measure disturbance data in the time and frequency domains. Finally, the micro-vibration reduction ratios were evaluated. From the experimental results, it is confirmed that the micro-vibration caused by the XAM motion can be reduced by about % on average as the 1σ value. The velocity optimization method proposed in this study is expected to be used efficiently in the jitter attenuation of the X-band antenna assembly. ICSV21, Beijing, China, July 214 6

7 21st International Congress on Sound and Vibration (ICSV21), Beijing, China, July 214 REFERENCES 1 Gerald, C. H., CCD Arrays Cameras and Displays, SPIE Press and JCD Publishing (1996). 2 Kim, D. K., Oh, S. H., Yong, K. L. and Yang, K. H., Numerical Study on a Reaction Wheel and Wheel-Disturbance Modeling, Journal of the Korean society for aeronautical & space sciences, 38(7), 72-78, (21). 3 Kim, D. K., Oh, S. H., Lee, S. H. and Yong, K. L., An Experimental Study on Microvibration Measurement Methods of a Reaction Wheel, Transactions of the Korean society for noise and vibration engineering, 21(9), , (211). 4 Schmid, M., Yong, S. S. and Lee, S. G., Extremely Compact Two-Axis X-Band Antenna Assembly, The 13th European Space Mechanisms and Tribology Symposium, Session 2: Pointing & Development Mechanism, (29). Lee, J. H., Ahn, H. S., Wang, S. M. and Choi, A Study on Efficient Approximation for Low-orbit Satellites, Proceedings of the KSAS Annual Spring Conference, , (211). 6 Choi, S. J., Jung, O. C., Kang, C. H., Kim, Y. W. and Jung, D. W., An Algorithm to Eliminate Discontinuity for LEO Satellite, Proceedings of the KSAS Annual Spring Conference, 94-97, (29). 7 Kim, D. K,, Oh, S. H., Kim, H. B., Lee, W. B. and Yong, K. L., Micro-vibration Test of X- band Antenna with Two-Axis Gimbal System, Proceedings of the KSAS Annual Autumn Conference, , (21). ICSV21, Beijing, China, July 214 7