Ultra-low noise sensor for magnetic fields
First Claim
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.
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Accused Products
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.
34 Citations
27 Claims
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1. A sensor for magnetic fields comprising:
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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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A sensor for magnetic fields comprising:
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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. - View Dependent Claims (10, 11, 12, 13, 14, 15, 16)
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17. A sensor for magnetic fields comprising:
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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. - View Dependent Claims (18, 19, 20, 21, 22, 23, 24, 25)
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26. A method for measuring a magnetic field comprising:
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providing a resonant structure having a resonance frequency; attaching at least one magnet to the resonant structure to form a proof mass, wherein the proof mass is configured to move in response to the force of a magnetic field acting on the magnet; providing 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 so as to concentrate magnetic flux at a location of the roof mass; providing a solenoid coil surrounding the proof mass and configured as part of an electrical feedback loop; nulling the magnetic field at the location of the proof mass and at frequencies below a threshold frequency; amplifying the motion of the proof mass by operation of a mechanical resonance of the resonant structure; measuring the displacement of the proof mass; and computing from the displacement of the proof mass a magnitude of a component of the magnetic field at the resonance frequency. - View Dependent Claims (27)
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Specification