ESA GNC Conference Papers Repository

This paper presents the development of a multi-axis magnetically levitated momentum exchange device named reaction sphere. This system is a magnetic bearing electromagnetic spherical actuator that consists in an 8-pole permanent magnet spherical rotor and a stator equipped with 20 coils. The stator coils are distributed on a sphere outside of the rotor and their position corresponds to the vertexes of a dodecahedron while the 8 rotor poles are distributed on the vertexes of a cube. Current in coils is controlled by a set of programmable sources. Using different types of currents profiles, the sphere located inside the coils can be levitated and torqued in any direction providing attitude stabilization for all three axes of the spacecraft with a single device. The development of the reaction sphere is performed in the frame of an FP7 programme. The consortium is led by CSEM (CH), and comprises maxon motor (CH), SENER (SP), SABCA (BE), CBK (PL), and RedShift (BE). The rotor and stator electromechanical design optimization is performed using COMSOL Multiphysics together with a developed hybrid FEM-analytical model and relies on specifications derived from the Proba-3 mission. For a total targeted mass of 6.5 kg and a stator diameter of 75 mm, the reaction sphere will provide an angular momentum of 1.4 Nms and a torque of 44 mNm. The maximum rotation speed is targeted to be at 4,000 rpm. Thanks to the selected rotor and stator pole arrangement, force and torque control are decoupled. Control algorithms for magnetic levitation and angular velocity rely on a classical state-space approach. The reaction sphere is instrumented with nine flux sensors and three optical position sensors. Flux sensors are used to determine the reaction sphere rotor orientation and its angular velocity. Position sensors are used to position the rotor inside the stator (magnetic levitation).