ACCELEROMETER-ONLY CALIBRATION METHOD

• US 20110077891A1
• Filed: 09/25/2009
• Published: 03/31/2011
• Est. Priority Date: 09/25/2009
• Status: Abandoned Application

• Current Assignee: CSR Technology Holdings Inc. (Qualcomm, Inc.)
• Original Assignee: SiRF Technology Holdings Incorporated (Qualcomm, Inc.)
• Inventors: KOENIG, Michael
• Application Number:
  US12/567,478
• Publication Number:
  US 20110077891A1
• Time in Patent Office: Days
• US Class Current: 702/104
• CPC Class Codes: G01P 21/00 Testing or calibrating of apparatus or devices covered by the preceding groups

Abstract

Methods for calibrating an accelerometer without needing to use external assistance signals, such as GNSS signals. The invention is applicable to accelerometers generally—in both GPS navigation devices and other devices. A method of calibrating an accelerometer, the accelerometer having a bias, comprises: rotating the accelerometer, wherein the orientation of the axis of the accelerometer changes with respect to the local gravity vector; collecting accelerometer measurements as the accelerometer is moving; and calculating the bias and/or scale factor; wherein the accelerometer measurements are primarily due to local gravity. Furthermore, some embodiments of the present invention include a self-calibration process—a process that does not engage the user'"'"'s attention. For example, in a self-calibration process for an accelerometer in a cell phone the moving step may be the movement of the phone out of a user'"'"'s pocket to hold it up to view the screen.

Claims

1. A method for calibrating an accelerometer without needing to use external assistance signals, said accelerometer having a bias, said bias being a zero offset for measurements by said accelerometer, said method comprising:

moving said accelerometer, wherein the orientation of the axis of said accelerometer changes with respect to the local gravity vector;

collecting accelerometer measurements as said accelerometer is moving; and

calculating said bias;

wherein said accelerometer measurements are primarily due to local gravity.

2. A method as in claim 1, wherein said collecting is periodic.

3. A method as in claim 2, wherein said collecting is at 10 Hz.

4. A method as in claim 1, wherein said collecting includes of the order of 100 measurements.

5. A method as in claim 1, further comprising weighting each of said measurements by an estimated noise power.

6. A method as in claim 1, wherein said accelerometer is a component accelerometer of an accelerometer triad.

7. A method as in claim 6, wherein said calculating includes solving the equation

G²=(A_X−

b_X)²+(A_Y−

b_Y)²+(A_Z−

b_Z)²

wherein A_x, A_y and A_z are measurements for the three component accelerometers of said accelerometer triad, b_x, b_y and b_z are corresponding biases, and G is a constant.

8. A method as in claim 1, wherein said solving includes a recursive method.

9. A method as in claim 1, further comprising calculating a scale factor of said accelerometer.

10. A method as in claim 9, wherein said accelerometer is a component accelerometer of an accelerometer triad.

11. A method as in claim 10, wherein said solving includes solving the equation

12. A method as in claim 1, wherein said moving said accelerometer is rotating said accelerometer.

13. A method as in claim 1, wherein said accelerometer is embedded in a portable device.

14. A method as in claim 13, wherein said moving said accelerometer is a self-calibration process involving no deliberate calibration motions by a user of said portable device.

15. A method as in claim 13, wherein said portable device is a communication device.

16. A method as in claim 13, wherein said portable device is a navigation device.

17. A method as in claim 13, wherein said accelerometer is a component accelerometer of an accelerometer triad, and wherein said self-calibration process involves motion of less than 20 degrees about any of the axes of said accelerometer triad.

18. A method as in claim 13, wherein said accelerometer measurements are due to local gravity and acceleration of said portable device, and wherein said acceleration of said portable device has a magnitude of less than 0.01 times local gravity.

19. A method as in claim 13, wherein said accelerometer measurements are due to local gravity and acceleration of said portable device, and wherein said acceleration of said portable device has a magnitude of less than 0.1 times local gravity.

20. A circuit for calibrating an accelerometer triad without needing to use external assistance signals, each component accelerometer of said accelerometer triad having a bias, said bias being a zero offset for measurements by said component accelerometer, said circuit comprising:

an analog to digital converter for converting analog accelerometer measurements from said accelerometer triad into digital accelerometer measurements;

a power routine processor block for weighting each of said digital accelerometer measurements by an estimated noise power; and

a bias estimate processor block for calculating said biases from weighted digital accelerometer measurements.

21. A circuit as in claim 20, wherein said biases are calculated by solving the equation

G²=(A_X−

b_X)+(A_Y−

b_Y)²+(A_Z−

b_Z)²

wherein A_x, A_y and A_z are measurements for the three component accelerometers of said accelerometer triad, b_x, b_y and b_z are corresponding accelerometer biases, and G is a constant.

22. A circuit as in claim 20, wherein said biases are calculated by solving the equation

23. A circuit as in claim 22, wherein said bias estimate processor block is also for calculating said scale factors.