US Patent 7557568 Actuator with position detecting mechanism

A position detecting mechanism for specifying the position of a movable body using magnetism includes a magnetic device mounted to the movable body and a magnetic sensor mounted to a housing. The magnetic device includes a pair of yokes made of a magnetic substance, a plurality of permanent magnets interposed between the yokes, and a holder for holding the magnets. The magnets are disposed in such a manner that the north and south magnetic poles are in contact with the yokes on both sides.