Practical Results on the Development of a Control Moment Gyro based Attitude Control System for Agile Small Satellites

In this paper a new practical Attitude Control System is proposed, based on Control Moment Gyroscopes (CMG). These actuators can provide unique torque, angular momentum and slew rate capabilities to small satellites without any increase in power, mass or volume. This will help small satellites become more agile and maneuverable. Agility considerably increases the operational envelope and efficiency of spacecraft and substantially increases the return of earth and science mission data. The paper focuses on the practical work on developing the hardware for a low cost, miniature CMG for agile small satellites. Experimental results indicate the potential benefits of using CMGs. Specifically, a cluster of four Single Gimbal CMGs (SGCMG) is used to practically demonstrate full 3-axis control for a microsatellite class spacecraft. Additionally, results are presented on the development of a larger SGCMG proposed as an experimental payload for future enhanced microsatellite missions. Introduction: A Single Gimbal CMG (SGCMG) is a CMG with a constant speed momentum wheel, gimbaled in one axis only. For full three-axis control of a spacecraft, a cluster of four CMGs is normally used. CMGs, due to their inherit gyroscopic properties can potentially generate large torque and angular momentum outputs, in a more efficient way than current technologies such as reaction or momentum wheels. Depending on the gimbal axes a CMG can be distinguished to a Single Gimbal CMG (SGCMG) and Double Gimbal CMG (DGCMG). The type and number of CMGs that can be used in an ACS is a trade off between performance, cost, mechanical and algorithm complexity. SGCMGs and Variable Speed CMGs (VSCMG) are the most powerful (from the torque point of view) of all, but SGCMGs require a minimum of four units for full 3-axis control in order to avoid singularities. SGCMGs have been thoroughly studied in the past and have been baselined to be used in future space missions. The most significant drawback with SGCMGs is the problem of singularities. This is the condition in which no torque can be produced for certain sets of gimbal angles. When the gimbal angles encounter singularities they ‘lock-up’, thus not being able to complete a commanded maneuver. The problem of singularities stresses the need to develop a steering logic, which will steer the CMG system away from these singularities with the minimum resources and within the system hardware (gimbal motor) constraints. Many laws have been developed in order to accommodate the problem of singularities with varying degrees of success 4-9, . Most of these have been developed for 4-SGCMGs in a pyramid configuration. A 4-SGCMG system of pyramid arrangement gives the advantage of having a spherical momentum envelope, which results to an almost equal momentum capability in all three axes. Satellite Reference Frame z