HUBBLE SPACE TELESCOPE ON-ORBIT SYSTEM IDENTIFICATION

摘要

The Hubble Space Telescope on-orbit experiences low frequency oscillations due to thermal excitation of the deployed solar arrays and possibly other appendages/equipment. Although these disturbances are small compared with standard spacecraft requirements, they cause higher than expected jitter levels for the HST. The modifications to the pointing control system initially utilized the frequencies and gain factors predicted by math models. Under normal practice, for system identification on the ground, the vehicle (or a very precise structural dynamic simulator) testing occurs in a deployed configuration using a wide range of well-understood methods for force input, data processing and post-test model correlation methods. Such a test was indeed performed for the HST, but did not include some appendages, such as the deployed Solar Arrays, due to gravitational effects. This uncertainty in the plant model at low frequencies affects the stability and performance of the control law if the control bandwidth increases. The HST On-orbit Transfer Function Test was intended to provide more accurate modal parameters for the low frequency modes used in the control law update. In addition, since a source of the on-orbit disturbance was a possibly stuck mechanism, and knowing that the modal frequencies were changing based on Solar Array pre-load and thermal load effects, testing afforded a method to periodically check the state of the vehicle on-orbit. The objective of the HST On-orbit Transfer Function Test was to measure the transfer function from the reaction wheels (torque input) to the rate gyros (angular rate sensor) and thereby characterize the modal parameters of the low frequency appendage modes of the HST which are important for control system design. A specially designed torque input pulse is applied to the vehicle via the reaction wheels and the response measured. The vehicle response and the torque input are processed using parameter estimation techniques to compute the transfer function and quantify the modal parameters. Development of forcing function type and amplitude are discussed and results compared with pre-test analyses and on-orbit data. Uncertainty bounds for the parameters are studied since the parameters vary based on orbit location, vehicle geometry induced non-linearities, control law coupling, gyro noise and measurement errors. The results are compared with pre-test analyses, math model simulations with the modal parameters and actual vehicle response data. The report concludes with recommendations for dynamic tests to characterize spacecraft on-orbit. The test was done under the guidance of the NASA as part of the HST project.