Electronic compass and compensation of large magnetic errors for operation over all orientations

• US 6,543,146 B2
• Filed: 12/06/2000
• Issued: 04/08/2003
• Est. Priority Date: 12/06/2000
• Status: Expired due to Term

• Current Assignee: Honeywell International Inc.
• Original Assignee: Honeywell International Inc.
• Inventors: Smith, Robert B., Morton, Blaise Grayson, Elgersma, Michael R.
• Primary Examiner(s): Gutierrez, Diego
• Assistant Examiner(s): Courson, Tania C.
• Application Number:
  US09/731,177
• Publication Number:
  US 20020100178A1
• Time in Patent Office: 853 Days
• Field of Search: 333/56, 333/55.R, 333/57, 333/58, 333/59, 333/61
• US Class Current: 33/356
• CPC Class Codes: G01C 17/38 Testing, calibrating, or compensating of compasses

Abstract

A three-axis algebraic model is used to numerically compensate for magnetic errors in measured magnetic field values in an electronic compass for any orientation of the compass. This model is based on physical principles and uses a linear algebra approach that facilitates computation of the parameters needed for compensation. During a calibration procedure of the electronic compass, magnetic and gravity fields are measured in three axes at each of a variety of combinations of orientations and azimuths. This set of measured magnetic and gravity fields is used to calculate a matrix compensation coefficient and a vector compensation coefficient using a system of equations. These compensation coefficients are stored and then used during normal operations of the electronic compass to correct all subsequently measured magnetometer data to obtain corrected values for the Earth'"'"'s magnetic field, from which the correct azimuth can be calculated.

Claims

1. A method of compensating an electronic compass to obtain accurate azimuth data for any orientation of the electronic compass despite the presence of perturbing magnetic effects, the method comprising:

obtaining a measured magnetic field vector H_MEAS representing magnetic field strength along three axes and a measured gravity vector G_MEAS representing gravitational field strength along the same three axes;

using a matrix compensation coefficient L_E and a vector compensation coefficient H_PE to correct measured magnetic field data by applying a formula of the form Ĥ

_EARTH≡

L_E·

H_MEAS−

H_PE, where Ĥ

_EARTH is the compensated value for the Earth'"'"'s true magnetic field; and

calculating the azimuth data as a function of the corrected magnetic field data.

2. The method of claim 1, further comprising, before using the matrix compensation coefficient L_E and the vector compensation coefficient H_PE to correct measured magnetometer output data to obtain the accurate magnetic field data:

obtaining, for each of a plurality of combinations of orientations and azimuths, a measured magnetic field vector H_MEAS representing magnetic field strength along three axes and a measured gravity vector G_MEAS representing gravitational field strength along three axes; and

calculating, as a function of the plurality of measured magnetic field vectors H_MEAS and the plurality of measured gravity vectors G_MEAS, a matrix compensation coefficient L_E and a vector compensation coefficient H_PE by solving a system of equations.

3. The method of claim 2, wherein the plurality of measured magnetic field vectors H_MEAS and the plurality of measured gravity vectors G_MEAS are measured at four orientations for each of four azimuths, the orientations and azimuths being substantially evenly distributed in all angles.

4. The method of claim 3, wherein the four orientations are substantially similar for each of the four azimuths.

5. The method of claim 2, further comprising storing a plurality of component values of the matrix compensation coefficient L_E and of the vector compensation coefficient H_PE in a memory associated with the electronic compass.

6. The method of claim 2, wherein the system of equations comprises a plurality of equations of the form

7. The method of claim 2, further comprising deriving a full system of equations by rotating Ĥ

_EARTH for each of the plurality of measured magnetic field vectors H_MEAS into a north/east/down coordinate frame of reference.

8. The method of claim 2, wherein the system of equations comprises a first subset of linear equations.

9. The method of claim 8, further comprising:

solving the first subset of linear equations to calculate an estimate of the matrix compensation coefficient L_E and an estimate of the vector compensation coefficient H_PE; and

using the estimate of the matrix compensation coefficient L_E and the estimate of the vector compensation coefficient H_PE to solve the full system of equations to calculate the matrix compensation coefficient L_E and the vector compensation coefficient H_PE.

10. The method of claim 9, further comprising iteratively solving the system of equations to refine the calculated matrix compensation coefficient L_E and the calculated vector compensation coefficient H_PE.

11. A method of compensating an electronic compass to obtain accurate azimuth data for any orientation of the electronic compass despite the presence of perturbing magnetic effects, the method comprising:

obtaining, for each of a plurality of combinations of orientations and azimuths, a measured magnetic field vector H_MEAS representing magnetic field strength along three axes and a measured gravity vector G_MEAS representing gravitational field strength along three axes;

calculating, as a function of the plurality of measured magnetic field vectors H_MEAS and the plurality of measured gravity vectors G_MEAS, a matrix compensation coefficient L_E and a vector compensation coefficient H_PE by solving a system of equations;

storing a plurality of component values of the matrix compensation coefficient L_E and of the vector compensation coefficient H_PE in a memory associated with the electronic compass;

subsequently retrieving the plurality of component values of the matrix compensation coefficient L_E and of the vector compensation coefficient H_PE from the memory;

obtaining a measured magnetic field vector H_MEAS representing magnetic field strength along three axes and a measured gravity vector G_MEAS representing gravitational field strength along three axes;

using the matrix compensation coefficient L_E and the vector compensation coefficient H_PE to correct measured magnetic field data by applying a formula of the form Ĥ

_EARTH≡

L_E·

H_MEAS−

H_PE, where Ĥ

_EARTH is the compensated value for the Earth'"'"'s true magnetic field; and

calculating the azimuth data as a function of the corrected magnetic field data.

12. An electronic compass arrangement, comprising:

a three-axis magnetometer, configured to obtain a measured magnetic field vector H_MEAS representing magnetic field strength along three axes;

a three-axis gravity sensor arrangement, configured to obtain a measured gravity vector G_MEAS representing gravitational field strength along three axes;

a microprocessor arrangement, communicatively coupled to the three-axis magnetometer and to the three-axis gravity sensor arrangement and configured to

use a matrix compensation coefficient L_E and a vector compensation coefficient H_PE to correct measured magnetic field data by applying a formula of the form Ĥ

_EARTH≡

L_E·

H_MEAS−

H_PE, where Ĥ

_EARTH is the compensated value for the Earth'"'"'s true magnetic field, and

calculate the azimuth data as a function of the corrected magnetic field data; and

a memory, communicatively coupled to the microprocessor arrangement and configured to store the matrix compensation coefficient L_E and the vector compensation coefficient H_PE.

13. The electronic compass arrangement of claim 12, wherein:

the three-axis magnetometer is further configured to obtain, for each of a plurality of combinations of orientations and azimuths, a measured magnetic field vector H_MEAS representing magnetic field strength along three axes;

the three-axis gravity sensor arrangement is further configured to obtain, for each of the plurality of combinations of orientations and azimuths, a measured gravity vector G_MEAS representing gravitational field strength along three axes; and

the microprocessor arrangement is further configured to, before using the matrix compensation coefficient L_E and the vector compensation coefficient H_PE to correct measured magnetometer output data to obtain the accurate magnetic field data, calculate, as a function of the plurality of measured magnetic field vectors H_MEAS and the plurality of measured gravity vectors G_MEAS, a matrix compensation coefficient L_E and a vector compensation coefficient H_PE by solving a system of equations.

14. The electronic compass arrangement of claim 13, wherein the plurality of measured magnetic field vectors H_MEAS and the plurality of measured gravity vectors G_MEAS are measured at four orientations for each of four azimuths, the orientations and azimuths being substantially evenly distributed in all angles.

15. The electronic compass arrangement of claim 13, wherein the system of equations comprises a plurality of equations of the form

16. The electronic compass arrangement of claim 13, wherein the microprocessor arrangement is further configured to derive a full system of equations by rotating Ĥ

_EARTH for each of the plurality of measured magnetic field vectors H_MEAS into a north/east/down coordinate frame of reference.

17. The electronic compass arrangement of claim 13, wherein the system of equations comprises a first subset of linear equations.

18. The electronic compass arrangment of claim 17, wherein the microprocessor arrangement is further configured to:

solve the first subset of linear equations to calculate an estimate of the matrix compensation coefficient L_E and an estimate of the vector compensation coefficient H_PE; and

use the estimate of the matrix compensation coefficient L_E and the estimate of the vector compensation coefficient H_PE to solve the full system of equations to calculate the matrix compensation coefficient L_E and the vector compensation coefficient H_PE.

19. The electronic compass arrangement of claim 18, wherein the microprocessor arrangement is further configured to iteratively solve the system of equations to refine the calculated matrix compensation coefficient L_E and the calculated vector compensation coefficient H_PE.

20. A microprocessor-readable medium having microprocessor-executable instructions stored thereon, the microprocessor-executable instructions causing a microprocessor to, upon execution of the microprocessor-executable instructions:

obtain a measured magnetic field vector H_MEAS representing magnetic field strength along three axes and a measured gravity vector G_MEAS representing gravitational field strength along three axes;

use a matrix compensation coefficient L_E and a vector compensation coefficient H_PE to correct measured magnetic field data by applying a formula of the form Ĥ

_EARTH≡

L_E·

H_MEAS−

H_PE, where Ĥ

_EARTH is the compensated value for the Earth'"'"'s true magnetic field; and

calculate the azimuth data as a function of the corrected magnetic field data.

21. The microprocessor-readable medium of claim 20, wherein the microprocessor-executable instructions further cause the microprocessor to, upon execution of the microprocessor-executable instructions and before using the matrix compensation coefficient L_E and the vector compensation coefficient H_PE to correct measured magnetometer output data to obtain the accurate magnetic field data:

obtain, for each of a plurality of combinations of orientations and azimuths, a measured magnetic field vector H_MEAS representing magnetic field strength along three axes and a measured gravity vector G_MEAS representing gravitational field strength along three axes; and

calculate, as a function of the plurality of measured magnetic field vectors H_MEAS and the plurality of measured gravity vectors G_MEAS, a matrix compensation coefficient L_E and a vector compensation coefficient H_PE using a system of equations.

22. The microprocessor-readable medium of claim 21, wherein the plurality of measured magnetic field vectors H_MEAS and the plurality of measured gravity vectors G_MEAS are measured at four orientations for each of four azimuths, the orientations and azimuths being substantially evenly distributed in all angles.

23. The microprocessor-readable medium of claim 21, wherein the system of equations comprises a plurality of equations of the form

24. The microprocessor-readable medium of claim 21, further comprising microprocessor-executable instructions causing a microprocessor to, upon execution, derive a full system of equations by rotating Ĥ

_EARTH for each of the plurality of measured magnetic field vectors H_MEAS into a north/east/down coordinate frame of reference.

25. The microprocessor-readable medium of claim 21, wherein the system of equations comprises a first subset of linear equations.

26. The microprocessor-readable medium of claim 25, further comprising microprocessor-executable instructions causing the microprocessor to, upon execution:

solve the first subset of linear equations to calculate an estimate of the matrix compensation coefficient L_E and an estimate of the vector compensation coefficient H_PE; and

use the estimate of the matrix compensation coefficient L_E and the estimate of the vector compensation coefficient H_PE to solve the full system of equations to calculate the matrix compensation coefficient L_E and the vector compensation coefficient H_PE.

27. The microprocessor-readable medium of claim 26, further comprising microprocessor-executable instructions causing the microprocessor to, upon execution, iteratively solve the system of equations to refine the calculated matrix compensation coefficient L_E and the calculated vector compensation coefficient H_PE.