Underactuated Attitude Control of Small Satellites Using Two CMGs
- Paper number
IAC-06-B5.6.17
- Author
Ms. Congying Han, Surrey Space Centre, University of Surrey, United Kingdom
- Coauthor
Dr. Alexandre Pechev, Surrey Space Centre, University of Surrey, United Kingdom
- Coauthor
Mr. Yoshi Hashida, Surrey Space Centre, University of Surrey, United Kingdom
- Year
2006
- Abstract
Control Moment Gyroscopes (CMGs) have been recently used on small satellites for attitude stabilization, fast slew-rate maneuvers and precise pointing. Despite the numerous advantages these actuators offer, CMGs suffer two serious flaws: 1) due to their complicated structure and additional movable parts in comparison to traditional momentum wheels, CMGs are susceptible to easier failure. This leads to the necessity for hardware redundancies. Typical full-scale configurations include four CMGs skewed at 54.73 degrees to provide three-axis control. Due to the strict limitation on mass, power and cost of small satellites, this redundancy is not desirable; 2) CMGs suffer from singularity, i.e. a demanded torque cannot be generated in the required direction. Redundant CMG configurations provide means for avoiding singularities by choosing alternative ”paths” to steer away from singular combinations. The number of singular points, however, is proportional to the number of CMGs used in the configuration. In this paper we study underactuated control laws providing three-axis attitude stabilization using only two CMGs. By involving the nonlinear coupling term in the dynamics, the acceleration component on the underactuated axis is used as a control input. In the work presented here, we investigate the controllability of a spacecraft system with two CMGs and analyze the necessary and sufficient conditions for the underactuated attitude control. A control law based on Lyapunov method is presented that, under some constraints, drives the system to a stable equilibrium. Simulation results are included to demonstrate three-axis attitude control of spacecraft using only two CMGs. Limitations of the proposed technique are also discussed. In addition, this paper provides singularity avoidance algorithms that employ gimbal positions commands instead of traditional gimbal rates commands. Hardware platform that is under development in the Surrey Space Centre (SSC) for on-ground underactuated control using CMGs is also presented.
- Abstract document