High-stability attitude control of satellites by magnetic bearing wheels

This paper presents an attitude control law for astronomy or earth-observat ion satellites, which require highly stable attitude-pointing for observati on and large-angle attitude maneuverability between successive observations . In the control law, magnetic bearing wheels (MBWs) are used instead of co nventional ball bearing wheels (BBWs). MBWs, whose rotors are magnetically suspended and thus have no mechanical contact, are low "microvibration" act uators for spacecraft attitude control systems. "All-axes-actively-controll ed" MBWs, just as in a control-moment gyro (CMG), provide the capability of tilting the rotational axis besides the rotor-speed control, whose allowab le tilt angle, however, is small (typically less than 3 degrees or so). In the proposed control law, multiple MBWs (which represent at least three for three axes control and preferably four for increased performance and hardw are redundancy) of this type are adopted as actuators of attitude control. The capability of rotor tilting is applied for broadening control bandwidth to improve the pointing performances while maintaining stability of the co ntrol system. The rotational control of the wheels are used for the purpose of 1) accommodating for the excessive angular momentum (= rotor-tilt-angle increments) that may otherwise result in too much tilting of the rotor to cause rotor touchdown, and also 2) large-angle maneuvers of spacecraft atti tude. Moreover, the increased degrees of control freedom of MBWs are advant ageously used for a further decrement of rotor-tilt angle. The mathematical formulation of our proposed control law is presented, and the results of t he numerical simulation on the control performance are also shown.