The Research On Active Control For Whole Spacecraft Vibration Isolation

Launch vehicles impart high levels of vibration to spacecraft during launch, which is well beyond the requirements for on-orbit performance. Launch dynamics are a major design driver in structural design of spacecraft. The objective of the whole spacecraft vibration isolation is to improve the dynamic environment of spacecraft during launch. This will increase launch reliability and reduce risk of equipment or component failure, as well as allow more sensitive equipment to be included in missions and reduce spacecraft structural weight and cost. This thesis is based on the application of the whole spacecraft isolation, in order to investigate the cubic configuration platform for vibration isolation and design several controllers for a six-axis isolator. The main contributions are as follows:First, a six degree of freedom active isolator based on Stewart platform has been designed for the purpose of whole spacecraft active vibration isolation. A detailed discussion is given, concentrating on the cubic architecture, the general kinematics and dynamic modeling and closed-loop behavior of the ideal Stewart platform. Piezoelectric actuator and geophone sensor techniques are analyzed. The 5th order mathematical modeling can be assumed for the purpose of the controller design.Secondly, the difference between passive isolation and active isolation are studied. Passive isolation is high reliability. However, once the design is determined the structure will not be changed. The design of a passive damper involves a trade-off between the resonance amplification and the high frequency attenuation. To maintain the sharp roll-off at high frequency while decreasing the overshoot at he resonance, active control is needed. A comparison between the sky-hook damper, integral force feedback and inertial velocity feedback is conducted. It has been determined that the performance gained by the active isolation system will be dependent on the stiffness of the launch vehicle. The launch vehicle stiffness becomes an important design parameter when developing a whole-spacecraft launch isolation system.Finally, various control techniques have been designed to control this isolator in a decentralized manner in the broadband. Based on the analyses of singular value, it can be assumed that the six inputs and six outputs were decoupled. Initially, simple single-input/single-output controllers such as lead controllers and notch filters were implemented for active vibration isolation. Then H∞controller was designed. The simulation results with these controllers confirm vibration attenuation remarkable.