Modified Stewart Platform for Spacecraft Thruster Vector Control

Abstract

Adaptive or intelligent structures which have the capability for sensing and responding to their environment promise a novel approach to satisfying the stringent performance requirements of future space missions. This paper introduces an intelligent modified Stewart platform as an adaptive thruster mount structure with precision positioning and active vibration suppression capabilities for use in space satellites as an intelligent thruster vector control platform. The intelligent thruster mount would utilize piezoelectric sensors and actuators for precision positioning and active vibration suppression to provide fine-tuning of position tolerance for thruster alignment and low transmissibility of vibration to the satellite structure. Similar intelligent platform, introduced here, may be used for sensitive equipment aboard of the spacecraft to suppress the vibration that resonates throughout the spacecraft structure during a thruster firing, solar panel boom opening/reorientation, etc. This vibration renders sensitive optical or measurement equipment non-operational until the disturbance has dissipated. This intelligent system approach would greatly enhance mission performance by fine tuning attitude control, potentially eliminating the non-operational period as well as minimizing fuel consumption utilized for position correction. The configuration of the intelligent thruster mount system is that of a modified Stewart platform. This system is an intelligent tripod with two in-plane rotational degrees of freedom (2-DOF) for the top device-plate. Precision positioning of this structure is achieved using active members that extend or contract to tilt the upper device-plate where the thruster is mounted. An inverse analysis of a modified Stewart platform is employed to determine the required axial displacement of the active struts for the desired angular tilt of the upper device-plate. The active struts can participate in precision positioning as well as vibration suppression of the upper device-plate where the thruster, i.e., the source of the unwanted vibrations and misalignment, is mounted. The proposed Thruster Vector Control (TVC) intelligent platform offers a promising method for achieving fine tuning of positioning tolerances of a thruster as well as minimizing the effects of the disturbances generated during thruster firing in spacecraft such as a satellite.