Steering Law of Control Moment Gyroscopes for Agile Attitude Maneuvers

What is it about?

This study proposes the CMG steering law to improve the agility performance for large-angle, and rest-to-rest multitarget maneuvers. Although CMGs can produce large torques as compared with the torques produced by reaction wheels (RWs) or momentum wheels (MWs), both practical and theoretical issues remain, such as the singularity problem. Some existing CMG steering methods for attitude maneuvers involve initial gimbal reorientation before slewing and settling. Reorientation is performed to move gimbals to desirable initial angles and to reduce the occurrence of singularities. While the time required to complete gimbal reorientation is shorter than that to complete slewing, the additive gimbal reorientation time would be impossible to ignore if the maneuvers were repeated and would hinder multitarget observations. Thus, it is useful to develop a suitable control/steering law to reduce or eliminate gimbal reorientation time.

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Why is it important?

To reduce total maneuver time, a steering law that not only provides the required attitude control torque but also controls the gimbals to the final target gimbal angles, is proposed for conventional CMG (not VSCMG). Until each maneuver is complete, the gimbals are steered to their final target gimbal angles, which are identical to the initial gimbal angles for the subsequent maneuver, thereby eliminating the need for gimbal reorientation between maneuvers. The steering law that do not include gimbal reorientation will enable a spacecraft to continuously perform agile multitarget maneuvers. However, in other works, the behavior of gimbals has not been investigated from the perspective of repeatability. In Sec. II. A, this Note presents a combined torqued- and null-motion steering law that is based on a singularity-robust inverse steering law, as well as a metric to evaluate the contribution of null motion with respect to the total gimbal rate. In Sec. II.B, sets of final target gimbal angles are determined with the condition of rest-to-rest maneuvers and the stability of gimbal angle control is analyzed using a Lyapunov function. A quadratic form, in accordance with the magnitude of null motion, is proposed to evaluate whether null motion can effectively control the gimbal angle. This analysis clearly explains the mechanisms of gimbal angle convergence and shows that gimbals may not converge to their final target angles when the quadratic form becomes too small. In Sec. III, to resolve the aforementioned repeatability issue, the steering law is modified by including a weighting matrix that depends on the prescribed attitude control torque profile. Finally, in Sec. IV, a total of 66 numerical simulations for agile multitarget maneuvers are conducted to verify the overall effectiveness and performance of the proposed steering law. Some performance metrics related to singularity identify sets of desirable final gimbal angles that improve the performances resulting from conventional steering laws.