Feedback control law for variable speed control moment gyros
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abstract
Variable speed control moment gyroscopes (CMG) are single-gimbal gyroscopes where the fly wheel speed is allowed to be variable. The equations of motion of a generic rigid body with several such variable speed CMGs attached are presented. The formulation is such that it can easily accommodate the classical cases of having either control moment gyros or reaction wheels to control the spacecraft attitude. A globally asymptotically stabilizing nonlinear feedback control law is presented. For a redundant control system, a weighted minimum norm inverse is used to determine the control vector. This approach allows the variable speed control moment gyroscopes to behave either more like classical reaction wheels or more like control moment gyroscopes, depending on the local optimal steering logic. Where classical control moment gyroscopes control laws have to deal with singular gimbal angle configurations, the variable speed control moment gyroscopes are shown not to encounter any singularities for many representative examples considered. Both a gimbal angle velocity and an acceleration-based steering law are presented. Further, the use of the variable speed CMG null motion is discussed to reconfigure the gimbal angles to preferred sets. Having a variable reaction wheel speed allows for a more general redistribution of the internal momentum vector.
