ESA GNC Conference Papers Repository
Development of Low Disturbance Magnetic Bearing Wheel (MBW) with Inclined Magnetic Poles
Recently, there has been an increase in the lifetime of satellites deployed in observation missions. The pointing requirements for satellites have also become more stringent. Almost all satellites are equipped with ball bearing wheels, which serve as actuators in the attitude control system. In general, friction and frictional wear due to mechanical contact of the ball bearings are inevitable; hence, the lifetime of the ball bearing wheel is limited. Furthermore, the ball bearing wheel has been identified as one of the disturbance sources that prevent satellites from having high pointing accuracy due to rotor imbalance and imperfections in the ball bearing structure. The magnetic bearing wheel (MBW), on the other hand, has certain advantages such as long lifetime of the bearing due to no mechanical contact, no stiction, and low wheel disturbance due to the implementation of rotor spin around the principal axis of inertia; thus, the use of the MBW is expected to overcome the problems of the ball bearing wheel . However, the fabrication of an MBW system requires electromagnets and rotor displacement sensors as well as control electronics; hence, its complex mechanism and heavy weight are perceived as disadvantages. Hence, we developed an MBW with inclined magnetic poles. This allows us to fabricate a 5-DOF magnetic bearing with six electromagnets and six displacement sensors, leading to a reduction in the size and weight of the system. In general, it is difficult to control the electromagnetic forces of magnetic bearings with inclined magnetic poles. Therefore, we have proposed a simple magnetic bearing controller, whose design is based on a geometric analysis of electromagnetic forces and magnetic bearing forces/moments. This magnetic bearing controller utilizes the redundancy of electromagnetic forces in order to satisfy certain restrictions on the magnetically suspended system and to optimize the electromagnetic forces used for biasing. However, the magnetic bearing controller cannot prevent those fluctuations in the electromagnetic forces that are caused by periodic changes in the air gaps in electromagnets and displacement sensors due to errors in the machining and assembly of the magnetic bearing; as a result, the MBW induces synchronous and harmonic disturbances in the system. With regard to this problem, various low-disturbance control methods have been proposed . However, unless the exact number of air gaps in the electromagnets and displacement sensors used in the rotor spin around the principal axis of inertia are obtained, these methods cannot result in a sufficient reduction in the disturbance. In order to reduce the disturbance caused by the MBW, we have developed a disturbance feedback controller. This controller generates commands to control the rotor displacement, thereby reducing the MBW disturbance. The magnetic bearing control system with the disturbance feedback controller has extremely good stability as the stability can be controlled by applying an appropriate phase lead to the displacement commands. This control system enables the MBW disturbance to be decreased to 0.1 Nrms/0.05 Nmrms or less for all rotor speeds up to 6000 rpm.