Multiple Landmark Acquisition and Tracking Using Variable Speed and Classical CMGs On-board Spacecraft

Abstract

It is well known that the high torque output of CMGs is affected by its geometric singularities. In Variable Speed Control Moment Gyros (VSCMGs), the changing wheel speed gives an extra degree of freedom per CMG to avoid or escape singular conditions. The effectiveness of utilizing a cluster of VSCMGs for multiple landmark acquisition and tracking has not been covered in literature. This is investigated in this paper and compared with the performance of a constant speed CMG cluster. For the same target sequence and a time optimal outer-loop attitude controller, roof-type 4-CMG clusters with constant or variable wheel speed are used to acquire and track multiple targets. The efficacy of the actuators under various steering laws is compared based on the metrics- normalized torque and momentum errors, singularity index, residual attitude errors, feasibility of realizing the computed gimbal rates and total energy consumed. The performance CMG and VSCMG steering laws with and without null motion are investigated. The CMG singular direction avoidance steering law with null motion gives enhanced time of payload operation (by 2 seconds), gives smoother gimbal rates, produces 62:5% less momentum error and consumes 13% less energy relative to the VSCMG steering with null motion.