RESONANT MEMS LORENTZ-FORCE MAGNETOMETER USING FORCE-FEEDBACK AND FREQUENCY-LOCKED COIL EXCITATION

• US 20140049256A1
• Filed: 12/28/2012
• Published: 02/20/2014
• Est. Priority Date: 07/25/2012
• Status: Active Grant

• Current Assignee: Silicon Laboratories Incorporated
• Original Assignee: Silicon Laboratories Incorporated
• Inventors: Smith, Eric B., Wahby, Riad, Zhou, Yan
• Granted Patent: US 9,588,190 B2
• Time in Patent Office: Days
• US Class Current: 324/259
• CPC Class Codes: B81B 3/0032 Structures for transforming energy not provided for in groups B81B3/0021 - B81B3/0029
• CPC Class Codes: G01R 33/0286 comprising microelectromechanical systems [MEMS] MEMS devices in general B81B

Abstract

A method includes supplying a current to at least one conductive path integral with a MEMS device to thereby exert a Lorentz force on the MEMS device in the presence of a magnetic field. The method includes determining the magnetic field based on a control value in a control loop configured to maintain a constrained range of motion of the MEMS device. The control loop may be configured to maintain the MEMS device in a stationary position. The current may have a frequency equal to a resonant frequency of the MEMS device.

Claims

1. A method comprising:

supplying a current to at least one conductive path integral with a microelectromechanical system (MEMS) device to thereby exert a Lorentz force on the MEMS device in the presence of a magnetic field; and

determining the magnetic field based on a control value in a control loop configured to maintain a constrained range of motion of the MEMS device.

2. The method, as recited in claim 1, wherein the control loop is configured to maintain the MEMS device in a stationary position.

3. The method, as recited in claim 1, wherein the current has a frequency approximately equal to a resonant frequency of the MEMS device (f_O—

_MEMS).

4. The method, as recited in claim 1, wherein the control value is based on sensed displacements of a proof mass of the MEMS device.

5. The method, as recited in claim 1, further comprising:

generating an indicator of a resonant frequency of the MEMS device (f<sub>O—

MEMS</sub>) in a first mode of operating the MEMS device; and

generating the current based on the indicator of f<sub>O—

MEMS</sub> in a second mode of operating the MEMS device,

wherein in the first mode the MEMS device is configured to resonate, and in the second mode, the MEMS device is included in the control loop configured to maintain the MEMS device in the stationary position.

6. The method, as recited in claim 5, wherein generating the indicator of f_O—

_MEMS comprises;

configuring the MEMS device in resonance to generate an oscillating signal; and

comparing a frequency of the oscillating signal to a frequency of a version of a reference clock signal and generating the indicator of f<sub>O—

MEMS</sub> based thereon.

7. The method, as recited in claim 5, wherein generating the current comprises:

adjusting a frequency of a version of a reference clock signal based on f<sub>O—

MEMS</sub>.

8. The method, as recited in claim 1, further comprising:

applying a force on the MEMS device in opposition to a difference between a frequency of the current and a resonant frequency of the MEMS device (f<sub>O—

MEMS</sub>),

wherein the frequency of the current is approximately f<sub>O—

MEMS</sub>.

9. The method, as recited in claim 1, wherein supplying the current comprises:

generating the current having the frequency equal to f<sub>O—

MEMS</sub> using a second MEMS device configured to self-resonate.

10. The method, as recited in claim 9, wherein generating the current comprises:

applying a force to the second MEMS device based on a difference between a frequency of the current and f<sub>O—

MEMS</sub>.

11. The method, as recited in claim 1, further comprising:

generating the current based on a reference clock signal; and

providing an output signal indicative of the magnetic field based on the control value and the reference clock signal.

12. The method, as recited in claim 1, further comprising:

generating the current based on a reference clock signal;

providing an output signal indicative of the magnetic field based on the sensed displacement and the reference clock signal; and

generating the control value based on the output signal and a phase-shifted version of the reference clock signal.

13. The method, as recited in claim 1, further comprising:

varying a frequency of the current over a range of frequencies; and

determining a frequency spectrum of a magnetic field having a bandwidth that exceeds a bandwidth of the MEMS device based on control values in the control loop used to maintain the MEMS device in a stationary position, the control values corresponding to frequencies of the range of frequency values of the current.

14. A magnetic sensor comprising:

a microelectromechanical system (MEMS) device including,

a mass suspended from a substrate;

at least one conductive path integral with the mass to allow a current to flow into and out of the mass and configured to exert a Lorentz force on the mass when a current flows through the conductive path in the presence of a magnetic field; and

a drive actuation transducer configured to apply a force to constrain a range of motion of the mass in response to a control signal indicative of the magnetic field.

15. The magnetic sensor, as recited in claim 14, wherein the force maintains the mass in a stationary position.

16. The magnetic sensor, as recited in claim 14, wherein the conductive path is configured to receive a current having a frequency locked to a resonant frequency of the MEMS device (f_O—

_MEMS).

17. The magnetic sensor, as recited in claim 14, further comprising:

a mixer configured to generate an output signal indicative of the magnetic field based on the control signal and a periodic reference signal used to generate the current.

18. The magnetic sensor, as recited in claim 17, wherein the periodic reference signal has a frequency equal to a resonant frequency of the MEMS device (f_O—

_MEMS).

19. The magnetic sensor, as recited in claim 17, further comprising:

a second mixer configured to generate the control signal based on the output signal and a phase-shifted version of the periodic reference signal used to generate the current.

20. The magnetic sensor, as recited in claim 14, further comprising:

a control loop including the MEMS device, the control loop being configured to maintain the mass in a stationary position.

21. The magnetic sensor, as recited in claim 14, further comprising:

a reference signal generator configured to generate a periodic reference signal having a frequency equal to a resonant frequency of the MEMS device (f<sub>O—

MEMS</sub>).

22. The magnetic sensor, as recited in claim 21, wherein the reference signal generator comprises:

a second MEMS device having approximately a same resonant frequency as the MEMS device and configured to resonate at the resonant frequency and generate the periodic reference signal having a frequency equal to the resonant frequency.

23. The magnetic sensor, as recited in claim 21, wherein the reference signal generator comprises:

a reference signal generator configured to generate a clock signal having a reference frequency,

wherein the microelectromechanical system (MEMS) device further includes a drive actuation transducer configured to apply a required force to change the resonant frequency to the reference frequency.

24. A method comprising:

supplying a current to at least one conductive path integral with a MEMS device to thereby exert a Lorentz force on the MEMS device in the presence of a magnetic field;

varying a frequency of the current over a range of frequencies; and

determining a frequency spectrum of a time-varying magnetic field having a bandwidth that exceeds a bandwidth of the MEMS device based on control values in a control loop, individual frequency components of the frequency spectrum of the time-varying magnetic field corresponding to individual frequencies of the range of frequencies.

25. The method, as recited in claim 24, wherein the control value is used to maintain the MEMS device in a stationary position.

26. The method, as recited in claim 24, wherein the control value is used to maintain the MEMS device in resonance.