US Patent 7898121 Linear actuator

A linear actuator includes permanent magnet annuli arranged about an armature core for axial movement in a tubular stator upon energization of coils arranged in concentric association with the armature. The stator has portions extending radially inwards of the coils and towards one another beneath each coil, which define a spacing between the coil and the armature. The annuli have a substantially radially magnetized structure and the coils are configured for single phase power input. In one embodiment (FIG. 6), two pairs of spaced annuli are arranged on the core, wherein the axial length of the outermost annuli is half the axial length of the inner annuli.