Apparatus and method for calibrating MEMS inertial sensors
The transduction scale factor for a MEMS gyroscope is calibrated without moving the MEMS device based on measurements of the resonator resonance frequency and the accelerometer resonance frequency as well as a distance value that may be a fixed distance value or a measured distance value. The measured distance value may be obtained by measuring the quality factor of the resonator or accelerometer and deriving the measured distance value from the quality factor measurement.
1. A method for calibrating a MEMS gyroscope having a resonator and an accelerometer, the resonator including at least one shuttle that resonates in a device plane above an underlying substrate and deflects out-of-plane, the accelerometer configured to sense such out-of-plane motion, the method comprising:
measuring a resonance frequency of the resonator;
measuring a resonance frequency of the accelerometer; and
producing a transduction scale factor value for the MEMS gyroscope based on the resonance frequency of the resonator and the resonance frequency of the accelerometer without physical movement of the gyroscope to measure electrical outputs produced in response to known movements of the gyroscope.
2. A method according to claim 1 , wherein measuring the resonance frequency of the resonator comprises:
inducing movement of the at least one shuttle within the device plane.
3. A method according to claim 1 , wherein measuring the resonance frequency of the accelerometer comprises:
inducing out-of-plane movement of the at least one shuttle.
4. A method according to claim 1 , wherein producing the transduction scale factor value comprises:
producing the transduction scale factor value based on the resonance frequency of the resonator, the resonance frequency of the accelerometer, and a fixed distance value representing the nominal distance between the at least one shuttle and corresponding accelerometer sense electrodes on the underlying substrate.
5. A method according to claim 1 , wherein producing the transduction scale factor value comprises:
measuring a distance between the at least one shuttle and corresponding accelerometer sense electrodes on the underlying substrate to produce a measured distance value; and
producing the transduction scale factor value based on the resonance frequency of the resonator, the resonance frequency of the accelerometer, and the measured distance value.
6. A method according to claim 5 , wherein measuring the distance comprises:
measuring a quality factor of the resonator; and
producing the measured distance value based on the measured quality factor of the resonator.
7. A method according to claim 5 , wherein measuring the distance comprises:
measuring a quality factor of the accelerometer; and
producing the measured distance value based on the measured quality factor of the accelerometer.
8. A method according to claim 1 , further comprising:
storing the transduction scale factor value in a memory of the MEMS device.
9. A method according to claim 1 , further comprising:
comparing the transduction scale factor value with a transduction scale factor value previously stored in a memory of the MEMS device.
10. A method according to claim 9 , further comprising:
determining, based on the comparison, whether the MEMS device passes or fails a test; and
producing an output indicating whether the MEMS device passes or fails the test.
11. A calibration device for calibrating a MEMS gyroscope having a resonator and an accelerometer, the resonator including at least one shuttle that resonates in a device plane above an underlying substrate and deflects out-of-plane, the accelerometer configured to sense such out-of-plane motion, the calibration device comprising:
a first frequency measurement subsystem configured to measure a resonance frequency of the resonator;
a second frequency measurement subsystem configured to measure a resonance frequency of the accelerometer; and
a transduction scale factor generator configured to produce a transduction scale factor value for the MEMS gyroscope based on the resonance frequency of the resonator and the resonance frequency of the accelerometer without physical movement of the gyroscope to measure electrical outputs produced in response to known movements of the gyroscope.
12. A calibration device according to claim 11 , wherein the first frequency measurement subsystem is configured to measure the resonance frequency of the resonator by inducing movement of the at least one shuttle within the device plane.
13. A calibration device according to claim 11 , wherein the second frequency measurement subsystem is configured to measure the resonance frequency of the accelerometer by inducing out-of-plane movement of the at least one shuttle.
14. A calibration device according to claim 11 , wherein the transduction scale factor generator is configured to produce the transduction scale factor value based on the resonance frequency of the resonator, the resonance frequency of the accelerometer, and a fixed distance value representing the nominal distance between the at least one shuttle and corresponding accelerometer sense electrodes on the underlying substrate.
15. A calibration device according to claim 11 , wherein the transduction scale factor generator is configured to measure a distance between the at least one shuttle and corresponding accelerometer sense electrodes on the underlying substrate to produce a measured distance value and produce the transduction scale factor value based on the resonance frequency of the resonator, the resonance frequency of the accelerometer, and the measured distance value.
16. A calibration device according to claim 15 , wherein the transduction scale factor generator includes a quality factor measurement subsystem for measuring a quality factor of the resonator and producing the measured distance value based on the measured quality factor of the resonator.
17. A calibration device according to claim 15 , wherein the transduction scale factor generator includes a quality factor measurement subsystem for measuring a quality factor of the accelerometer and producing the measured distance value based on the measured quality factor of the accelerometer.
18. A calibration device according to claim 11 , wherein the transduction scale factor generator is configured to store the transduction scale factor value in a memory of the MEMS device.
19. A calibration device according to claim 11 , wherein the transduction scale factor generator is configured to compare the transduction scale factor value with a transduction scale factor value previously stored in a memory of the MEMS device.
20. A calibration device according to claim 19 , wherein the transduction scale factor generator is further configured to determine, based on the comparison, whether the MEMS device passes or fails a test, and to produce an output indicating whether the MEMS device passes or fails the test.