High-stability attitude control of satellites by magnetic bearing wheels

Abstract

This paper presents an attitude control law for astronomy or earth-observation satellites, which require highly stable attitude-pointing for observation and large-angle attitude maneuverability between successive observations. In the control law, magnetic beating wheels (MBWs) are used instead of conventional ball bearing wheels (BBWs). MBWs, whose rotors are magnetically suspended and thus have no mechanical contact, are low "microvibration" actuators for spacecraft attitude control systems. "All-axes-actively-controlled" MBWs, just as in a control-moment gyro (CMG), provide the capability of tilting the rotational axis besides the rotor-speed control, whose allowable 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 hardware 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 control 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 attitude. Moreover, the increased degrees of control freedom of MBWs are advantageously used for a further decrement of rotor-tilt angle. The mathematical formulation of our proposed control law is presented, and the results of the numerical simulation on the control performance are also shown.