A 3(5) Degree of Freedom Electrodynamic-Bearing Wheel for 3-Axis Spacecraft Attitude Control Application

This paper deals with two related types of electrodynamically suspended rotors, acting either as momentum or reaction wheels for spacecraft attitude control purposes. One is of the three-axis active type, the other involves five actively controlled degrees of freedom. The wheels are of a pancake design with the control forces acting on the outer rim where the main rotor mass is concentrated thus avoiding low structural resonances. The rim is decoupled from the hub by titanium springs. Both wheels provide a vernier gimballing capability enabling three-axis attitude control of spacecraft with one rotating mass only. A special decoupling tilt control loop design eliminates all gyroscopic effects, such as precessional and nutational oscillations. The electrodynamic bearing principle is advantageous because main parts of the mass can be concentrated on the outer rim of the rotor contributing to the desired inertia. Force generation is linear involving only very small lags and practically no cross-coupling exist between the orthogonal axes facilitating a control strategy to prevent disturbing unbalanced forces from affecting the stator of the bearing for the use in so-called 'micro-gravity' environment. The wheels are actually in the development phase and in the paper the principles, the design, the control loops and first results are presented.