Multi-Axis Chip-Scale MEMS Inertial Measurement Unit (IMU) Based on Frequency Modulation

• US 20140208823A1
• Filed: 01/28/2013
• Published: 07/31/2014
• Est. Priority Date: 01/28/2013
• Status: Active Grant

• Current Assignee: Regents of the University of California (University of California)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Trusov, Alexander A., Zotov, Sergei A., Shkel, Andrei M.
• Granted Patent: US 9,274,136 B2
• Time in Patent Office: Days
• US Class Current: 73/1.38
• CPC Class Codes: G01C 19/5783 Mountings or housings not specific to any of the devices covered by groups G01C19/5607 - G01C19/5719
• CPC Class Codes: G01P 15/097 by vibratory elements
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/18 in two or more dimensions
• CPC Class Codes: G01P 21/00 Testing or calibrating of apparatus or devices covered by the preceding groups

Abstract

A multi-axis microelectromechanical-systems (MEMS) inertial measurement unit (IMU) is fabricated in a vacuum sealed single packaged device. An FM vibratory gyroscope and an FM resonant accelerometer both for generating FM output signals is fabricated in the silicon chip using MEMS. A signal processor is coupled to the an FM vibratory gyroscope and to the FM resonant accelerometer for receiving the FM gyroscopic output signals and the FM accelerometer output signals. The signal processor generates simultaneous and decoupled measurement of input acceleration, in put rotation rate, and temperature and/or temperature distribution within the IMU, self-calibration of the biases and scale factors of the IMU and its support electronics against temperature variations and other common mode errors, and reduction of the cross axis sensitivity by reducing acceleration errors in the gyroscope and rotation errors in the accelerometer.

Claims

1. A multi-axis microelectromechanical-systems (MEMS) inertial measurement unit (IMU) in a vacuum sealed single packaged device comprising:

a single silicon chip;

an FM vibratory gyroscope for generating frequency modulated (FM) gyroscopic output signals fabricated in the silicon chip using silicon MEMS technologies as part of the vacuum sealed single packaged device;

an FM resonant accelerometer for generating frequency modulated (FM) accelerometer output signals fabricated in the silicon chip using silicon MEMS technologies as part of the vacuum sealed single packaged device; and

a signal processor coupled to the an FM vibratory gyroscope and to the FM resonant accelerometer for receiving the frequency modulated (FM) gyroscopic output signals and the frequency modulated (FM) accelerometer output signals, the signal processor generating simultaneous and decoupled measurement of input acceleration, input rotation rate, and temperature and/or temperature distribution within the IMU, self-calibration of the biases and scale factors of the IMU and its support electronics against temperature variations and common mode errors, and reduction of the cross axis sensitivity by reducing acceleration errors in the gyroscope and rotation errors in the accelerometer.

2. The MEMS IMU of claim 1 where the FM vibratory gyroscope generates a differential FM signal including two splitting frequencies with inherent self-calibration against temperature and common mode errors, where the sum of the two splitting frequencies is proportional to the sensor temperature and the difference between the two splitting frequencies provides a measure of the input rate of rotation.

3. The MEMS IMU of claim 1 where the FM resonant accelerometer generates a differential FM signal including two splitting frequencies with inherent self-calibration against temperature and common mode errors, where the sum of the two splitting frequencies is proportional to the sensor temperature and the difference between the two splitting frequencies is a measure of the input acceleration.

4. The MEMS IMU of claim 1 where FM resonant accelerometer comprises:

a tuning fork resonator having two tines with two degrees of freedom, such as an anti-phase and in-phase mode of vibration of the two tines; and

variable gap parallel plate electrodes coupled to the two tines to produce a negative electrostatic spring, where external acceleration causes a shift of both tines with in-phase motion, which changes an effective gap in the parallel plate electrodes thereby changing the effective stiffness for each tine, where the change of stiffness induces a change of an anti-phase resonant frequency, and where the change of the anti-phase resonant frequency is an FM measure of the input acceleration.

5. The MEMS IMU of claim 4 where the accelerometer includes two sets of variable gap parallel plate electrodes having opposite orientations so that interchangeable operation of the two sets of parallel plate electrodes reverses the polarity of an accelerometer scale factor, enabling self-calibration of bias drift by means of a differential measurement arising from the interchangeable operation of the two sets of parallel plate electrodes.

6. The MEMS IMU of claim 4 where the two tines are momentum balanced so that the anti-phase mode of vibration generates quality factors above 1,000,000 and frequency stability below 1 ppb, enabling very good FM detection resolution and stability for the gyroscope and accelerometer.

7. The MEMS IMU of claim 4 where in-phase mode of vibration of the accelerometer is arranged and configured to have a low quality factor (for example, below 1,000) by viding energy dissipation through the chip or through the use of a feedback system to provide wide bandwidth and fast response time.

8. The MEMS IMU of claim 4 where the accelerometer include two tuning forks, each of the two tuning forks having the same sensitive axis, which is the axis of the anti-phase mode vibrations, but polar opposite scale factors by reversed orientation of the negative electrostatic spring produced by the parallel plate electrodes, where simultaneous and coordinated operation of the two tuning forks yields a differential FM accelerometer in which the two output frequencies are used to detect acceleration and sensor temperature at the same time, and to eliminate the effect of temperature and common mode errors on the accelerometer output.

9. The MEMS IMU of claim 1 where the FM gyroscope and FM accelerometer comprise separate x, y, z single-axis sensors or combination sensors with multiple axis of rotation sensitivity or acceleration sensitivity.

10. MEMS IMU of claim 1 where FM resonant accelerometer comprises:

a tuning fork resonator having two tines with two degrees of freedom, such as an anti-phase and in-phase mode of vibration of the two tines; and

an electrode arrangement coupled to the tines which has a capacitance as a nonlinear function of the displacement of the tines or a mechanical spring element with non-linear dependence of stiffness versus stress.

11. A method of operation and self-calibration of a frequency modulation inertial measurement unit (FM IMU) comprising:

continuously or periodically driving an FM accelerometer into anti-phase resonance by means of an electronic feedback system, which includes tracking of the resonant frequency;

generating a measure of input acceleration of the FM IMU from the driven FM accelerometer as an FM signal of the accelerometer;

demodulating the FM signal of the accelerometer to measure the input acceleration by using, for example, a phase locked loop (PLL), frequency counter, or zero crossing detection;

periodically switching a DC bias voltage applied to the FM accelerometer parallel plate electrodes, which have opposite orientations and which are coupled to the accelerometer tuning forks between two fixed values to periodically switch the accelerometer scale factor (in Hz/g) between two different output values of equal magnitude but opposite polarity of an output signal; and

signal processing the two output values of the output signal to decouple the effect of temperature and common mode errors from true acceleration, whereby intentional switching of the scale factor results in self-calibration of the FM accelerometer.

12. The method of claim 11 where each accelerometer includes two tuning forks and where intentional switching of the scale factor results in self-calibration of the FM accelerometer is applied to one tuning fork at a time or simultaneously to both tuning forks to provide continuous self-calibrated measurement of the true acceleration without interruption.

13. The method of claim 11 where the FM accelerometer is capable of amplitude modulated (AM) operation and further comprising switching the FM accelerometer between amplitude modulated (AM) mode of operation and FM mode by operation of a control system.

14. The method of claim 11 where the FM gyroscope is capable of amplitude modulated (AM) operation and further comprising switching the FM gyroscope between an FM rate measuring mode, AM rate measuring mode, and Whole Angle mode of operation by operation of a control system.

15. The method of claim 11 where continuously or periodically driving an FM accelerometer is implemented by velocity positive feedback (self-resonance), phase locked loop (PLL), or excitation using frequency band noise, impulse excitation.

16. The method of claim 11 where the FM vibratory gyroscope and FM resonant accelerometer each have sensitive axes, further comprising initially aligning the sensitive axes of the FM vibratory gyroscope and FM resonant accelerometer by a single lithography step when fabricated to provide high precision of the alignment between the sensitive axes and stability of the relative alignment of the sensitive axes over the environmental exposures and lifetime.

17. The method claim 11 further comprising simultaneously signal processing of the FM gyroscope signal and two complimentary FM accelerometers allows for decoupling and elimination of the acceleration and vibration induced errors in the FM gyroscope.

18. The method of claim 11 where signal processing comprises simultaneous signal processing multiple FM signals produced by the resonant accelerometers to provide temperature self sensing and self-calibration against temperature induced and common mode measurement errors.

19. A multi-axis microelectromechanical-systems (MEMS) inertial measurement unit (IMU) comprising:

a silicon chip;

an FM vibratory gyroscope capable of generating frequency modulated (FM) gyroscopic output signals fabricated in the silicon chip using silicon MEMS technologies;

an FM resonant accelerometer capable of generating frequency modulated (FM) and amplitude modulated (AM) accelerometer output signals fabricated in the silicon chip using silicon MEMS technologies; and

a signal processor coupled to the an FM vibratory gyroscope and to the FM resonant accelerometer for receiving the frequency modulated (FM) gyroscopic output signals and the frequency modulated (FM) accelerometer output signals, the signal processor generating decoupled measurements of input acceleration, input rotation rate, and temperature and/or temperature distribution within the IMU, self-calibration of the biases and scale factors of the IMU and its support electronics against temperature variations and common mode errors, and reduction of the cross axis sensitivity by reducing acceleration errors in the gyroscope and rotation errors in the accelerometer.

20. A method′

of operation and self-calibration of an frequency modulation inertial measurement unit (FM IMU) comprising;

driving an FM gyroscope;

driving an FM accelerometer into anti-phase resonance by means of an electronic eedback system, which includes tracking of the resonant frequency;

generating a measure of input acceleration of the FM IMU from the driven FM accelerometer as an FM signal of the accelerometer;

demodulating the FM signal of the accelerometer to measure the input acceleration; and

signal processing the two output values of the output signal to decouple the effect of temperature and common mode errors from true acceleration.