Patent ID: 7893689

Claim:
A displacement measuring device comprising: a movable magnetic member disposed in an accommodation space having opposite sides extending in parallel with each other and linked with a moving object to move in a longitudinal direction thereof; a magnetic sensor having a sensing surface disposed at one side of the accommodation space to confront the moving magnetic member at a first air gap thereby sensing magnetic flux; an axisymmetric stationary magnetic member for providing the accommodation space therein, said stationary magnetic member including a sensor supporting member supporting the magnetic sensor at the center thereof, a pair of yoke members axisymmetrically disposed at opposite sides of the sensor supporting member, a pair of permanent magnets each of which is disposed between one of the yoke members and the sensor supporting member, thereby providing a pair of magnetic circuits each of which crosses the movable magnetic circuit, wherein: the permanent magnets are polarized so that magnetic flux flows in one of the magnetic circuits is opposite in direction to magnetic flux flowing in the other magnetic circuit; and each of the yoke members has a projection having a surface extending along the other side of the accommodation space in parallel with the longitudinal surface of the movable magnetic member at a second air gap so that magnetic permeance of the first and second magnetic circuits changes when the moving magnetic member moves in the longitudinal direction.