Steering control law for double-gimbal scissored-pair CMG

Abstract

Because control moment gyroscopes (CMGs) can generate a large torque compared to reaction wheels, they are used as actuators for attitude control of large spacecraft. However, when the number of Single-Gimbal CMG (SGCMG) units is five or less, there can be internal singularities that cannot generate torque around the desired direction. To construct a system that has no internal singularities, six or more SGCMGs are required, because an orthogonally arrayed, three scissored-pair CMG system has no internal singularities. Because CMG singularities are disruptive for attitude control, a great deal of effort has been devoted to overcoming the CMG singularity problem; the various designs include Variable-Speed CMGs (VSCMGs), Double-Gimbal CMGs (DGCMGs), and Double-Gimbal Variable-Speed CMGs (DGVSCMGs). However, these designs still have problems, such as a slow response to torque generation commands about the wheel axis in VSCMGs and DGVSCMGs, and difficulty in precise attitude tracking when perturbation torque is generated to avoid singularities. To overcome the problems of the traditional CMG configurations, this paper proposes a new CMG system configuration that we call the Double-Gimbal Scissored-pair CMG (DGSPCMG) system. Because the DGSPCMG system is a hybrid system combining a Scissored-Pair CMG and a DGCMG, the DGSPCMG system does not have internal singularities except at the origin and along the x-axis. Moreover, this system can recover from an internal singularity by null motion only, and from outer singularities (saturation singularities) by steering the scissored-pair gimbals only. Thus, generation of perturbation torque is unnecessary for recovering from singularities, and a precise attitude tracking maneuver can be more easily achieved. This paper presents a conceptual design of a DGSPCMG system and describes a steering control law for the proposed system. Furthermore, the validity of the proposed steering control law is demonstrated through numerical simulations and results of comparison experiments are shown to demonstrate the advantage of the DGSPCMG over a VSCMG.