Reduction of Unbalanced Magnetic Forces in Planar Linear Actuators

To reach a high axial thrust force density with planar linear motors slotted designs have advantages compared to the common air-gap winding topologies. Combined with a permanent magnet excitation compact solutions can be designed, which exhibit a significant force perpendicular to the direction of movement. Even with double sided topologies a destabilizing stiffness remains. This force adds up an extra load on the bearings and can significantly limit their life-time especially when bush bearings are applied. This paper introduces a concept to actively compensate for the permanent magnet induced bearing load. The potential and limits are analyzed based on a planar linear actuator with E-shaped stator cores and a four phase winding. It is outlined how an integrated active bearing force compensation can be reached for the complete axial stroke even with simple conventional actuator layouts. This method can (i) significantly reduce wear and thus improve lifetime and (ii) reduce the size of the mechanical bearings. The system simulation results are validated with measurements of a prototype system.