Determination of safety rotor displacement zone for active magnetic bearing reaction wheel

The article presents the general advantages of active magnetic bearings used as rotor supports for momentum/reaction wheels of spacecraft attitude control systems. The necessity of launch-lock device, providing rigid mechanical coupling of rotor and momentum/reaction wheel housing during transportation and launch of an orbiting vehicle, as well in the case of emergency condition, occurring during orbital operation, are substantiated. The article proposes launch-lock device equipped with emergency bearings, conical retainers, which are moved in axial direction under the attraction forces of the electromagnets and permanent magnets. For the proposed device, the design procedure for calculation of the required axial movement of the retainer is developed. On the one hand, this axial movement must be big enough to provide contactless rotor levitation in the magnetic field during normal mode operation. At the same time, axial displacement of retainer must preclude the possibility of mechanical contact of rotor and momentum/reaction wheel stator in the emergency state. The developed procedure considers various types of rotor displacement relative to its nominal position: translational motion, rotation around the transverse axis of the momentum/reaction wheel, as well as cases of simultaneous translational and angular movement. The procedure takes into account the dynamic behavior of spinning rotor in the magnetic field. It means the calculation of rotor linear oscillation along the transverse axis, generated by residual static imbalance, angular oscillations about this axis, caused by residual dynamic imbalance, and magnitude of the resultant oscillations. Furthermore, the procedure provides carrying out of forced oscillations analysis and modal analysis to obtain the rotor natural frequencies, their corresponding mode shapes and magnitudes of elastic deformations of spinning rotor, with a glance the stiffness coefficient of the active magnetic bearing system. The developed procedure makes possible to draw the conclusion about sufficiency of the axial movement value of the launch-lock retainer, verify the absence of the mechanical contact between spinning rotor and stators of the active magnetic bearing system and stators of another momentum/reaction wheel elements, for example, drive motor, rotor angular position sensor.