Ultra-low noise sensor for magnetic fields

Patent metadata

Field	Value
Publication number	US-10018686-B1
Application number	US-201514919527-A
Country	US
Kind code	B1
Filing date	Oct 21, 2015
Priority date	Oct 21, 2015
Publication date	Jul 10, 2018
Grant date	Jul 10, 2018

Abstract

An ultra-low noise sensor for magnetic fields comprises a mechanically resonant structure having a magnetized proof mass. The displacement of the proof mass due to a magnetic field provides a high resolution and highly amplified measurement of magnetic field fluctuations near the resonance frequency. A flux modulator may be used with the resonant structure to amplify magnetic fluctuations in a non-resonant frequency band. The resonant structure, combined with a high resolution readout device and a frequency-compensating numerical processor, can amplify magnetic fluctuations in a broad range of frequencies. A solenoid coil surrounding the resonant structure may be used to null the quasi-static earth's magnetic field and thereby increase the dynamic range of the sensor. Cryogenically cooling the resonant structure can improve the resolution of the sensor. A magnetometer that embodies features of the present invention is miniaturized and has improved amplification and resolution at room temperature.

First claim

What is claimed is:

1. A sensor for magnetic fields comprising:
a micromachined body;
at least one magnet attached to the body and, together with the body, forming a proof mass;
one or more micromachined flexures mechanically connected between the body and a substrate, wherein the proof mass and the flexures form a resonant structure having a high quality factor and a resonance frequency;
two pieces of magnetically permeable material, located on opposite sides of the proof mass, each at a separation distance from the proof mass, and configured to concentrate magnetic flux at a location of the proof mass;
a high resolution readout system having a level of input-referred readout noise, configured to provide an electrical output as a function of displacement of the proof mass;
a processor operatively connected to the readout system and having a frequency compensating transfer function; and
a solenoid coil surrounding the proof mass and configured as part of a feedback loop to null a magnetic field at the location of the proof mass and at frequencies below a threshold frequency.

2. The sensor of claim 1 , wherein the separation distance is chosen so as to allow the resonant structure to be designed with increased mechanical ruggedness, subject to the constraint that the resonance frequency has a specific value.

3. The sensor of claim 1 , further comprising a vacuum package that encloses the resonant structure.

4. The sensor of claim 1 , wherein the high resolution readout system comprises:
an optically reflective surface belonging to the proof mass;
a laser beam configured to illuminate the optically reflective surface of the proof mass and reflect from it, creating a reflected laser beam;
a prism configured to split the reflected laser beam into two split beams, so that the difference in power between the split beams is a function of the displacement of the proof mass; and
an optical detector configured to measure the difference in power between the split beams.

5. The sensor of claim 4 , wherein the optical detector is a bilateral split detector configured so that each split beam impinges on a different half of the split detector.

6. The sensor of claim 4 , wherein the optical detector is a knife edge detector.

7. The sensor of claim 4 , further comprising a vacuum package that encloses the resonant structure.

8. The sensor of claim 1 , wherein an electrical resonance frequency of the solenoid coil is chosen to minimize noise introduced by the solenoid coil into a measurement of the magnetic field.

9. A sensor for magnetic fields comprising:
a micromachined body;
at least one magnet attached to the body and, together with the body, forming a proof mass;
one or more micromachined flexures mechanically connected between the body and a substrate, wherein the proof mass and the flexures form a resonant structure having a high quality factor and a resonance frequency;
two pieces of magnetically permeable material, located on opposite sides of the proof mass, each at a separation distance from the proof mass, and configured to concentrate magnetic flux at a location of the roof mass;
a high resolution readout system having a level of input-referred readout noise, configured to provide an electrical output as a function of displacement of the proof mass, wherein the separation distance is chosen so as to maximize concentrator gain, subject to a constraint that an input-referred noise due to Brownian motion of the proof mass is below the level of input-referred readout noise; and
a processor operatively connected to the readout system and having a frequency compensating transfer function.

10. The sensor of claim 9 , further comprising a vacuum package that encloses the resonant structure.

11. The sensor of claim 9 , wherein the high resolution readout system comprises:
an optically reflective surface belonging to the proof mass;
a laser beam configured to illuminate the optically reflective surface of the proof mass and reflect from it, creating a reflected laser beam;
a prism configured to split the reflected laser beam into two split beams, so that the difference in power between the split beams is a function of the displacement of the proof mass; and
an optical detector configured to measure the difference in power between the split beams.

12. The sensor of claim 11 , wherein the optical detector is a bilateral split detector configured so that each split beam impinges on a different half of the split detector.

13. The sensor of claim 11 , wherein the optical detector is a knife edge detector.

14. The sensor of claim 11 , further comprising a vacuum package that encloses the resonant structure.

15. The sensor of claim 9 , further comprising a solenoid coil surrounding the proof mass and configured as part of a feedback loop to null a magnetic field at the location of the proof mass and at frequencies below a threshold frequency.

16. The sensor of claim 15 , wherein an electrical resonance frequency of the solenoid coil is chosen to minimize noise introduced by the solenoid coil into a measurement of the magnetic field.

17. A sensor for magnetic fields comprising:
a micromachined body;
at least one magnet attached to the body and, together with the body, forming a proof mass;
one or more micromachined flexures mechanically connected between the body and a substrate, wherein the proof mass and the flexures form a resonant structure having a high quality factor and a resonance frequency;
two pieces of magnetically permeable material, located on opposite sides of the proof mass, each at a separation distance from the proof mass, and configured to concentrate magnetic flux at a location of the proof mass;
a high resolution readout system having a level of input-referred readout noise, configured to provide an electrical output as a function of displacement of the proof mass;
a processor operatively connected to the readout system and having a frequency compensating transfer function, wherein the high resolution readout system comprises:
an optically reflective surface belonging to the proof mass,
a laser beam configured to illuminate the optically reflective surface of the proof mass and reflect from it, creating a reflected laser beam,
a prism configured to split the reflected laser beam into two split beams, so that the difference in power between the split beams is a function of the displacement of the proof mass, and
an optical detector configured to measure the difference in power between the split beams; and

a cryogenic cooling system, configured to cool the resonant structure and further configured so that any magnetic or conductive component of the cooling system is located at a sufficient distance from the resonant structure that the component does not increase noise in the electrical output.

18. The sensor of claim 17 , wherein the magnet comprises a high critical temperature superconducting material.

19. The sensor of claim 17 , wherein the separation distance is chosen so as to allow the resonant structure to be designed with increased mechanical ruggedness, subject to the constraint that the resonance frequency has a specific value.

20. The sensor of claim 17 , further comprising a vacuum package that encloses the resonant structure.

21. The sensor of claim 17 , wherein the optical detector is a bilateral split detector configured to so that each split beam impinges on a different half of the split detector.

22. The sensor of claim 17

Assignees

• Charles Stark Draper Laboratory Inc

Inventors

• Bickford James Alan
• Bernstein Jonathan Jay
• Weinberg Marc Steven
• Duwel Amy Elizabeth
• Elliott Iii Richard David
• Gagnon Jr Ronald Edmund
• Kinast Joseph Michael
• Leblanc John Joseph
• Marinis Jr Thomas Francis
• Tomaino-Iannucci Michael Joseph
• Trinkle Jr William Joseph

Classifications

• G01R33/0286 Primary

  comprising microelectromechanical systems [MEMS] (MEMS devices in general B81B) · CPC title

• G01R33/035

  using superconductive devices · CPC title

• G01R33/0047

  Housings or packaging of magnetic sensors (packaging of semiconductor devices H10W99/00); Holders · CPC title

• G01R33/038

  using permanent magnets, e.g. balances, torsion devices · CPC title