Abstract
Small-scale magnetically suspended control moment gyros with flat shaped rotor tilting by axial forces are easier to reduce size and weight than by radial forces. This paper presents a novel three-degree-of-freedom hybrid axial magnetic bearing without subsidiary air gaps. This new structure is more efficient because the loss on extra air gaps can be avoided. An order scheduling cross-feedback control strategy with order number adjusting with the rotor speed is also presented to compensate the gyroscopic effect. The damping ratios of nutation and precession are compared for cross-feedback controllers with different orders using generalized root locus with gain related to the speed of revolution. Simulation results show that, when increasing the order of the cross-feedback controller, the nutation stability rises, but precession drops. In experiment, high-order cross-feedback leads to high maximum stable speed, but collision at low speed. The solution is using first-order cross-feedback when the speed is below 6200 r/min, second order during 6200–9000 r/min, and third order above 9000 r/min. The digital control system implementing the presented control strategies on a field programmable gate array can keep the rotor in stable suspension from 0 to 12 000 r/min.
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