Combined Singularity Avoidance for Variable Speed Control Moment Gyroscope Clusters

摘要

Variable Speed Control Moment Gyroscopes (VSCMGs), defined by Schaub in (1), effectively applied to combined energy storage and attitude control in (2,3), and further studied by Yoon and Tsiotras in (4,5), can avoid attitude singularities by varying wheel speed. However, this device's key advantage lies in torque amplification via gimballing (i.e. operating in its CMG mode). The primary limitation of this CMG mode, though, is the existence of gimbal- lock singularity states. Avoiding escapable singularity states, studied extensively by Wie for CMGs (6) and Schaub for VSCMGs (7), is exacerbated in the case of simultaneous attitude and power tracking requirements (5). Although (5) gives an algorithm for combined energy storage and attitude control with VSCMG singularity avoidance, it does not directly evaluate this algorithm with respect to other robust CMG singularity avoidance methods from the literature. For this reason, two practical steering laws were adapted from previous results for this challenging case and compared to the results given in (5). First, Wie's Generalized Singularity Robust (GSR) steering law, originally developed for attitude tracking with CMGs (8) was modified to use VSCMGs. Although the GSR effectively perturbs gimbals through locked configurations when compared to classical CMG methods (see 6,9, 10), it also imparts small altitude torque errors which can linger for a few minutes after a CMG singularity. This problem can be disruptive to some space missions that use satellite off-pointing (e.g. precision earth imaging) where accurate trajectory tracking during operations is paramount.