Linearized magnetic displacement sensor

• US 6,515,474 B1
• Filed: 12/03/1999
• Issued: 02/04/2003
• Est. Priority Date: 08/06/1997
• Status: Expired due to Term

• Current Assignee: Fisher Controls International LLC (Emerson Electric Company)
• Original Assignee: Fisher-Rosemount Systems Incorporated (Emerson Electric Company)
• Inventors: Dilger, John P., Pepperling, Donald P., Dielschneider, Nile K.
• Primary Examiner(s): PATIDAR, JAY M
• Application Number:
  US09/454,801
• Time in Patent Office: 1,159 Days
• Field of Search: 324/207.24, 324/207.2, 324/207.22, 324/207.21, 324/207.23, 324/207.25, 324/207.26, 324/260, 324/262, 338/32.H, 338/32.R
• US Class Current: 324/207.24
• CPC Class Codes: G01D 5/145 influenced by the relative movement between the Hall device and magnetic fields see G01R33/06

Abstract

A displacement sensor includes a magnet assembly having a housing for mounting to one of first and second relatively moveable elements. A magnet assembly is mounted to one element and defines a longitudinal space having an axis along its length. The magnet assembly includes at least one magnet having a north and a south pole that supply fringing magnetic flux in the longitudinal space and a flux-shaping pole piece on each of the north and south poles. A magnetic field sensor assembly has a housing for mounting to the other of the first and second elements for movement along an axis in the space parallel to the magnet assembly. In one embodiment, each of the flux-shaping pole pieces has a pentagon shape in a plane of primary flux pattern, forming a first face confronting the longitudinal space between the first and second magnets. In another embodiment, the magnet is a cylindrical magnet adjacent the longitudinal space, and the first face of each flux-shaping pole piece is cylindrical and coaxial with the cylindrical magnet. Preferably, the pole piece has a frusto-conical face confronting the longitudinal space adjacent the cylindrical face.

Claims

1. A displacement sensor for sensing relative displacement along a substantially parallel path between first and second elements comprising:

a magnet assembly for mounting to the first element, the magnet assembly having a longitudinal space along and about a longitudinal axis of the first element parallel to the parallel path, the magnet assembly including

a magnet producing a generally radially symmetric magnetic field that is coaxil with the longitudinal axis of the first element, the magnet having a north pole and a south pole that supply magnetic flux throughout the longitudinal space, and

a flux-shaping pole piece on each of the north and south poles, the flux-shaping pole pieces having configurations to sculpt the magnetic flux so that magnetic flux density varies substantially linearly along the path; and

a magnetic flux sensor mounted to the second element for detecting variations in magnetic flux indicative of the relative displacement between the first and second elements.

2. The displacement sensor of claim 1, wherein each of the flux-shaping pole pieces is a metal pole piece appended to a respective pole of the magnet.

3. The displacement sensor of claim 2, wherein the pole pieces are attached to the respective pole by a magnetic force exhibited by the magnet.

4. The displacement sensor of claim 1, wherein each of the flux-shaping pole pieces is integral to the magnet.

5. The displacement sensor of claim 1, wherein the magnet is a bar magnet having a length with the poles at opposite ends thereof, wherein the longitudinal space is adjacent to and extends along the length of the bar magnet.

6. The displacement sensor of claim 1, wherein each of the flux-shaping pole pieces has a hexagon shape in a plane of a primary flux pattern in the longitudinal space, the hexagonal shape forming a first surface confronting the longitudinal space parallel to the axis of the longitudinal space.

7. The displacement sensor of claim 6, wherein each of the flux-shaping pole pieces is a metal pole piece appended to a respective pole of the magnet.

8. The displacement sensor of claim 1, wherein each of the poles has a pole face substantially perpendicular to the length of the magnet, and each of the flux-shaping pole pieces is a metal pole piece having a first surface confronting the longitudinal space and a second surface appended to a respective pole face, the second surface being perpendicular to the first surface, and the pole piece being oriented on the respective pole so that the first surface is parallel to the length of the magnet.

9. The displacement sensor of claim 8, including a third surface confronting the longitudinal space between the first and second surfaces, the third being acutely positioned from an axis parallel to the longitudinal space.

10. The displacement sensor of claim 8, wherein the magnet is a bar magnet having a length with the poles at opposite ends, wherein the longitudinal space is adjacent to and extends along the length of the bar magnet.

11. The displacement sensor of claim 10, wherein the bar magnet is cylindrical in shape and the first surface of each flux-shaping pole piece is cylindrical.

12. The displacement sensor of claim 10, wherein each of the flux-shaping pole pieces is a metal pole piece appended to a respective pole of the bar magnet.

13. The displacement sensor of claim 10, wherein the bar magnet has cylindrical shape and each of the poles has a circular pole face substantially perpendicular to the length of the cylindrical magnet, each of the flux-shaping pole pieces is a metal pole piece having a circular face appended to a respective circular pole face, the first surface of each flux-shaping pole piece is cylindrical and the perpendicular to the circular face, the pole piece being oriented on the respective pole so that the cylindrical first surface is parallel to and coaxial with the cylindrical bar magnet.

14. The flux-shaping pole piece of claim 13, including a frusto-conical surface confronting the longitudinal space between the cylindrical surface and the circular face.

15. The displacement sensor of claim 13, wherein each of the flux-shaping pole pieces is a metal pole piece appended to a respective pole of the cylindrical magnet.

16. The displacement sensor of claim 8, wherein one of the first and second elements is a valve actuator and valve stem housing and the other of the first and second elements is a stationary housing, whereby the displacement sensor senses linear displacement between the valve stem and the stationary housing.

17. The displacement sensor of claim 1, wherein one of the first and second elements is a valve actuator and valve stem housing and the other of the first and second elements is a stationary housing, whereby the displacement sensor senses linear displacement between the valve stem and the stationary housing.

18. The displacement sensor of claim 1, wherein the magnet assembly includes a magnet assembly housing and the magnet is mounted to the magnet assembly housing.

19. The displacement sensor of claim 1, wherein the magnet assembly includes a magnet assembly housing and the magnet is a cylindrical magnet mounted to the magnet assembly housing, the axis of the longitudinal space along which the magnetic flux density varies substantially linearly is adjacent substantially parallel to the length of the cylindrical magnet.

20. A flux-shaping pole piece for a pole of a magnet of a magnetic displacement sensor for sensing relative displacement along a substantially parallel path between first and second elements, wherein the magnet is to be arranged to form a longitudinal space in which the parallel path is substantially parallel to a length of the magnet, the pole piece comprising:

a magnetic material having an attachment surface arranged to attach to the pole of the magnet, and

a configuration so disposed and arranged so that when a pole piece is coaxially attached to the poles of the magnet each of the pole pieces sculpt magnetic flux in the longitudinal space so that magnetic flux density is generally radially symmetric through the longitudinal space and varies substantially linearly along the path.

21. The flux-shaping pole piece of claim 20, wherein the flux-shaping pole piece is a metal pole piece for appending to a pole of the magnet.

22. The flux-shaping pole piece of claim 21, wherein the pole piece is designed to be held in contact with the magnet pole by a magnetic force exhibited by the magnet.

23. The flux-shaping pole piece of claim 20, having a hexagonal shape in a plane of primary flux pattern across the longitudinal space arranged to form a first surface confronting the longitudinal space and parallel to the axis the longitudinal space when the pole piece is attached to the magnet pole.

24. The flux-shaping pole piece of claim 23, including a second surface confronting the longitudinal space between the first surface and the attachment surface.

25. The flux-shaping pole piece of claim 23, wherein the first surface is arranged parallel to the length of the magnet when the pole piece is attached to the magnet pole.

26. The flux-shaping pole piece of claim 25, wherein the attachment surface is perpendicular to the first surface, the attachment surface orienting the pole piece on the pole so that the narrow face is parallel to the length of the magnet.

27. The flux-shaping pole piece of claim 20, wherein the magnet is a cylindrical bar magnet having an axis, and the pole piece has first surface confronting the longitudinal space in a plane of primary flux pattern across the longitudinal space when the pole piece is attached to the magnet pole.

28. The flux-shaping pole piece of claim 27, wherein the flux-shaping pole piece is a metal pole piece and the first surface is cylindrical, the pole piece having a circular face perpendicular to the cylindrical first surface for attachment to a pole face of the magnet to orient the pole piece so that the cylindrical first surface is coaxial with the magnet.

29. The flux-shaping pole piece of claim 27, including a frusto-conical surface confronting the longitudinal space between the cylindrical surface and the circular face.

30. In a displacement sensor for sensing relative displacement along a substantially parallel path between first and second elements in which a magnet assembly arranged to be mounted to the first element to define a longitudinal space having a central axis parallel to the parallel path and a magnetic field sensor is arranged to be mounted to the second element in the longitudinal space so that relative movement between the first and second elements causes the magnetic field sensor to move relative to the magnet assembly along the central axis, the magnet assembly including:

a cylindrical bar magnet having a north pole and a south pole that supply magnetic flux in the longitudinal space, and

means on the poles to sculpt magnetic flux in the longitudinal space so that magnetic flux density is generally symmetrically distributed in planes approximately perpendicular to the central axis, the magnetic flux varying substantially linearly along the path in the presence of relative rotation about the central axis.