Winding layout for active bearing force reduction in tubular linear motors

Reaching a high surface thrust is for many applications that implement a direct linear motor actuation a key requirement. It allows compact solutions with a low mover mass, what is especially for oscillatory direct drives a main issue. Thus, in contrast to common air gap winding solutions slotted designs are favored. With tubular linear designs, the resulting radial forces ideally cancel, but a significant destabilizing radial stiffness remains. In the presence of an inevitable eccentricity of the mover, the unbalanced magnetic pull adds up an extra load on the bearings, limits efficiency and reduces the bearing lifetime. This is of special interest when bush bearings are applied. This paper proposes a concept that actively compensates for the resulting radial forces. Next to the winding system for thrust generation, an additional independent set of windings is installed to cancel radial forces. A systematic design of a proper compensation winding system is outlined and its advantages and limits are discussed. The proposed compensation has the potential to significantly reduce wear and thus improve efficiency and bearing lifetime.