Singularity Analysis and Avoidance of Variable-Speed Control Moment Gyros – Part II : Power Constraint Case

A Variable Speed Control Moment Gyro (VSCMG) is a relatively recent torque actuator device for spacecraft attitude control. As implied by its name, a VSCMG is a singlegimbal control moment gyro (CMG) with a flywheel having variable speed. Owing to this extra degree of freedom, a VSCMG can be used to achieve additional objectives, such as power tracking and/or singularity avoidance, as well as attitude control. In this article, a singularity analysis and a singularity avoidance method are proposed for the case when a VSCMG cluster is used as part of an integrated power/attitude control system (IPACS) onboard a satellite. The gimbal rates of the VSCMGs are used to provide the referencetracking torques, while the wheel accelerations are used both for attitude tracking and power reference tracking. The latter is achieved by storing or releasing the kinetic energy stored in the VSCMG flywheels. A null motion method to avoid singularities is also presented and a criterion is developed to determine the momentum region over which this method will successfully avoid singularities. This criterion can be used to size the wheels and develop appropriate momentum damping strategies tailored to specific mission requirements.