Micromechanical d'arsonval magnetometer

• US 5,731,703 A
• Filed: 10/31/1995
• Issued: 03/24/1998
• Est. Priority Date: 10/31/1995
• Status: Expired due to Fees

• Current Assignee: Charles Stark Draper Laboratory Incorporated
• Original Assignee: Charles Stark Draper Laboratory Incorporated
• Inventors: Bernstein, Jonathan J., Weinberg, Marc S.
• Primary Examiner(s): Snow, Walter E.
• Application Number:
  US08/550,923
• Time in Patent Office: 875 Days
• Field of Search: 324/244, 324/256, 324/259, 324/260, 324/261
• US Class Current: 324/256
• CPC Class Codes: G01R 33/02 Measuring direction or magnitude of magnetic fields or magnetic flux G01R33/20 takes precedence

Abstract

A micromechanical D'"'"'Arsonval magnetometer for sensing magnetic fields at low frequency with high sensitivity for operation near the resonant frequency of a micromechanical structure comprising a movable proof mass supported by torsion flexures, a conductive winding formed on the movable proof mass, at least one bridge electrode spanning the movable proof mass, a source for electrically biasing the movable proof mass relative to the bridge electrode(s), and a drive for electrically driving the conductive winding. Magnetic fields in the plane of the proof mass perpendicular to an axis of rotation formed by the torsion flexures interact with current passing through the conductive winding so as to torque the proof mass about the axis of rotation, whereby the resulting rotation is sensed through capacitors formed between the proof mass and the bridge electrode(s). The present invention micromechanical D'"'"'Arsonval magnetometer can be operated either open or closed loop. An open loop embodiment allows the proof mass to twist solely under the influence of the magnetic fields, while a closed loop embodiment restrains the proof mass motion. The loop can be closed by constructing additional electrodes or by using the existing bridge electrode(s) for both sensing and rebalancing.

Claims

1. A sensor for sensing low frequency magnetic fields with high sensitivity, said sensor comprising:

a proof mass, said proof mass being movable about an axis of rotation;

a conductive winding, said conductive winding being formed on said proof mass while being electrically isolated therefrom;

at least one bridge electrode, said at least one bridge electrode being disposed proximate said proof mass;

means for electrically biasing said proof mass relative to said at least one bridge electrode; and

means for electrically exciting said conductive winding.

2. The sensor as defined in claim 1, further comprising a substrate, wherein said proof mass is etched from said substrate, and wherein said axis of rotation is provided by at least two torsion flexures which remain between said proof mass and said substrate after said etching.

3. The sensor as defined in claim 2, wherein said substrate is silicon.

4. The sensor as defined in claim 1, wherein said proof mass has at least one stress relief aperture formed therein.

5. The sensor as defined in claim 1, wherein said proof mass is silicon.

6. The sensor as defined in claim 1, wherein said conductive winding is electrically isolated from said proof mass by an insulating layer formed therebetween.

7. The sensor as defined in claim 6, wherein said insulating layer is silicon oxide.

8. The sensor as defined in claim 1, wherein said conductive winding is gold.

9. The sensor as defined in claim 1, wherein said at least one bridge electrode comprises two bridge electrodes symmetrically spaced and located over opposite ends of said proof mass, equidistant from said axis of rotation.

10. The sensor as defined in claim 1, wherein said at least one bridge electrode is gold.

11. The sensor as defined in claim 1, wherein said at least one bridge electrode is electroplated.

12. The sensor as defined in claim 1, wherein said means for electrically biasing said proof mass comprises means for applying a DC bias signal thereto.

13. The sensor as defined in claim 1, wherein said means for electrically exciting said conductive winding comprises means for applying an AC drive signal thereto.

14. The sensor as defined in claim 13, wherein said proof mass has a resonant frequency, and wherein said AC drive signal is oscillating near said resonant frequency.

15. The sensor as defined in claim 14, wherein said AC drive signal is oscillating at 5% below said resonant frequency during open loop operation.

27. The sensor as defined in claim 1, wherein said proof mass and said at least one bridge electrode have holes formed therein for reducing Brownian motion effects.

30. The sensor as defined in claim 1, wherein said sensor is less than 1 mm² in size.

16. A sensor for sensing low frequency magnetic fields with high sensitivity, said sensor comprising:

a substrate;

a proof mass, said proof mass being movable about an axis of rotation provided by at least two torsion flexures formed between said proof mass and said substrate;

a conductive winding, said conductive winding being formed on said proof mass while being electrically isolated therefrom;

at least one bridge electrode, said at least one bridge electrode being disposed proximate said proof mass;

means for electrically biasing said proof mass relative to said at least one bridge electrode; and

means for electrically exciting said conductive winding.

17. The sensor as defined in claim 16, wherein said proof mass has a resonant frequency, and wherein said means for electrically exciting said conductive winding comprises means for applying an AC drive signal oscillating near said resonant frequency thereto.

18. The sensor as defined in claim 17, wherein said AC drive signal is oscillating at 5% below said resonant frequency during open loop operation.

19. The sensor as defined in claim 16, wherein said at least one bridge electrode comprises two bridge electrodes symmetrically spaced and located over opposite ends of said proof mass, equidistant from said axis of rotation.

28. The sensor as defined in claim 16, wherein said proof mass and said at least one bridge electrode have holes formed therein for reducing Brownian motion effects.

31. The sensor as defined in claim 16, wherein said sensor is less than 1 mm² in size.

20. A sensor for sensing low frequency magnetic fields with high sensitivity, said sensor comprising:

a proof mass, said proof mass being movable about an axis of rotation;

a conductive winding, said conductive winding being formed on said proof mass while being electrically isolated therefrom;

at least one bridge electrode, said at least one bridge electrode being disposed proximate said proof mass;

means for electrically biasing said proof mass relative to said at least one bridge electrode;

means for electrically exciting said conductive winding; and

means for sensing the distance between said at least one bridge electrode and said proof mass.

21. The sensor as defined in claim 20, further comprising:

at least one rebalance electrode, said at least one rebalance electrode being disposed proximate said proof mass; and

means for electrically driving said at least one rebalance electrode so as to maintain said proof mass in a relatively balanced position with respect to said at least one bridge electrode.

22. The sensor as defined in claim 21, wherein said means for electrically driving said at least one rebalance electrode provides an output signal indicative of a magnetic field applied to said sensor.

23. The sensor as defined in claim 21, wherein electrostatic force is used to maintain said proof mass in a relatively balanced position with respect to said at least one bridge electrode.

24. The sensor as defined in claim 21, wherein said means for electrically driving said at least one rebalance electrode utilizes said means for sensing the distance between said at least one bridge electrode and said proof mass so as to provide a closed loop feedback system.

25. The sensor as defined in claim 20, wherein said means for sensing the distance between said at least one bridge electrode and said proof mass provides an output signal indicative of a magnetic field applied to said sensor.

26. The sensor as defined in claim 20, wherein said means for sensing the distance between said at least one bridge electrode and said proof mass comprises means for electrically driving said at least one bridge electrode so as to maintain said proof mass in a relatively balanced position with respect to said at least one bridge electrode.

29. The sensor as defined in claim 20, wherein said proof mass and said at least one bridge electrode have holes formed therein for reducing Brownian motion effects.

32. The sensor as defined in claim 20, wherein said sensor is less than 1 mm² in size.