Automatic calibration of a three-axis magnetic compass

• US 7,451,549 B1
• Filed: 08/09/2006
• Issued: 11/18/2008
• Est. Priority Date: 08/09/2006
• Status: Expired due to Fees

• Current Assignee: PNI Corporation (PNI Digital Media)
• Original Assignee: PNI Corporation (PNI Digital Media)
• Inventors: Hsu, George, Oh, Becky, Sodhi, Raj, Prunty, Jay
• Primary Examiner(s): Bennett; G. Bradley
• Assistant Examiner(s): Johnson; Amy Cohen
• Application Number:
  US11/502,593
• Time in Patent Office: 832 Days
• Field of Search: 333/55.R, 333/56, 333/57, 702/85, 702/92, 702/93, 702/94, 702/17, 702/70, 702/190, 701/224
• US Class Current: 33/356
• CPC Class Codes: G01C 17/38 Testing, calibrating, or compensating of compasses

Abstract

A magnetic compass apparatus and method to account for magnetic distortion while determining a magnetic heading is disclosed. The method enables a compass module, comprising at least two magnetometers, to characterize its magnetic environment dynamically, while collecting data of a geomagnetic field; a user moves an apparatus through various orientations; the environment may or may not contain magnetic distortion influences. Data gathered by magnetometers and, optionally, accelerometers are processed through at least two filters before being transferred as a processed data set for repetitive measurement calculations. A series of calculations is executed recursively in time by solving one or more linear vector equations using processed data.

Claims

1. A method for determining a distortion corrected magnetic heading comprising the steps:

acquiring a first set of data from two or more magnetometers and a second set of data from one or more means for gravity field strength sensing within a predetermined time period in a first orientation;

processing the first set of data with a first digital filter;

processing the second set of data with a second digital filter;

compensating the first set of data with a first temperature compensation algorithm;

compensating the second set of data with a second temperature compensation algorithm;

generating a first filtered signal by processing the first filtered set of data with a first nonlinear function whose coefficients are predetermined;

generating a second filtered signal by processing the second filtered set of data with a second nonlinear function whose coefficients are predetermined; and

calculating a heading orientation from the first and second filtered signals, wherein a multiplicity of the first and second data sets are acquired in a multiplicity of orientations for the determining, such that the accuracy of the distortion correction improves as additional first and second data sets are acquired and wherein the acquiring, the processing, the compensating, the generating, the calculating and the determining are continuously ongoing.

2. A method for determining a distortion corrected magnetic heading as in claim 1 further comprising the steps:

calculating a pitch and roll orientation from said second filtered signal;

generating a third data set by processing said first filtered signal and said second filtered signal;

calculating a tilt compensated heading orientation from said third data set; and

communicating the tilt compensated heading orientation and the pitch and roll orientation.

3. A method for determining a distortion corrected magnetic heading as in claim 1 wherein said multiplicity of first and second data sets and said multiplicity of orientations is at least five.

4. A method for determining a distortion corrected magnetic heading as in claim 1 wherein said predetermined time period is less than about one second.

5. A method for determining a distortion corrected magnetic heading comprising the steps:

acquiring a first set of data from two or more magnetometers for magnetic field strength sensing and a second set of data from one or more means for gravity field strength sensing within a predetermined time period in a first orientation;

generating a first filtered data set by processing the first set of data with a first digital filter, wherein the first digital filter comprises a Finite Impulse Response algorithm;

generating a second filtered data set by processing the second set of data with a second digital filter, wherein the second digital filter comprises a Finite Impulse Response algorithm;

compensating the first filtered data set with a first temperature compensation algorithm;

compensating the second filtered data set with a second temperature compensation algorithm;

calibrating the first set of data with a first nonlinear function whose coefficients are predetermined;

calibrating the second set of data with a second nonlinear function whose coefficients are predetermined;

generating a third set of magnetometer data from the calibrated first and second set of data;

wherein the generating a third set of magnetometer data step comprises the steps;

filtering said calibrated first set of data with a stability filter;

filtering said calibrated second set of data with a minimum tilt separation filter;

processing the filtered first and second set of data with a median filter with a first-in-first-out buffer to define the third set of data;

processing the third set of data with a recursive least squares soft-iron and hard-iron offset estimator comprising a first stage estimator and a second stage estimator;

wherein said first-stage estimator comprises calculating an upper-triangular soft-iron distortion matrix and a hard-iron offset vector to obtain a first-stage corrected geomagnetic field vector and wherein said second-stage estimator comprises calculating a rotation matrix to obtain a second stage corrected geomagnetic field vector;

calculating a pitch and roll orientation from the calibrated second set of data;

calculating a heading orientation from the calibrated second set of data and the third set of magnetometer data, wherein a multiplicity of first and second data sets are acquired in at least five orientations for the determining such that the accuracy of the distortion correction improves with two or more first and second data sets acquired; and

communicating the magnetic heading and the pitch and roll orientation.

6. A method for determining a distortion corrected magnetic heading as in claim 5 wherein said communicating comprises an interface device comprising at least one from a group comprising visual display, visual touch display, optical fiber, electronic, audible, printed, telephonic, wireless and digital symbols.

7. A method for determining a distortion corrected magnetic heading as in claim 5 wherein said predetermined time period is less than about one second.

8. A method for determining a distortion corrected magnetic heading as in claim 5 wherein said two or more means for gravity field strength sensing are chosen from a group comprising gyroscopes, inclinometers and accelerometers.

9. A magnetic compass apparatus comprising:

two or more magnetometer sensors;

one or more means for gravity field strength sensing;

a magnetic distortion correction algorithm;

means for signal conditioning for the sensors wherein said means for signal conditioning comprises a finite-impulse-response digital filter with user-selective taps;

means for processing;

wherein the means for processing comprises at least one processor configured to apply a fitting function with parameters determined using least squares to correct a calibration setting for converting sensor data to gravity field strength readings and/or magnetometer readings; and

wherein the means for processing comprises at least one processor configured to process received magnetometer data with a stability filter, to process received gravity field strength data using a minimum-tilt-separation filter, and to process magnetometer data and gravity field strength data using a median filter with a first-in-first-out buffer to define a third set of data; and

wherein the means for processing comprises at least one processor configured to process the third set of data with a recursive least squares soft-iron and hard-iron offset estimator comprising a first stage estimator and a second stage estimator; and

wherein said recursive least squares soft-iron and hard-iron offset estimator further comprises predetermined or user selectable initialization values;

means for information storage storing at least a portion of the magnetic distortion correction algorithm;

at least one temperature compensator wherein the at least one temperature compensator processes magnetic sensor data;

a means for starting and stopping a calibration procedure; and

means for communicating.

10. The magnetic compass apparatus of claim 9, wherein said magnetometer sensors comprise magneto-inductive sensors.

11. The magnetic compass apparatus of claim 9, wherein said one or more means for gravity field strength sensing are chosen from a group comprising gyroscopes, inclinometers and accelerometers.

12. The magnetic compass apparatus of claim 9, wherein said means for information storage comprises at least one from a group comprising volatile semiconductor, non-volatile semiconductor and hard disk memory wherein at least one from a group comprising gravity field strength reading, magnetometer reading, one or more sets of calibration settings, one or more sets of calculated pitch, roll, and tilt-compensated heading readings, initialization values, filtering algorithm, location code and identity code are stored therein.

13. The magnetic compass apparatus of claim 9, wherein said means for communicating comprises at least one interface device chosen from a group comprising visual display, visual touch display, keyboard, signals via hard wire, Bluetooth™

, video, voice, RF, optical fiber, telephonic, wireless and digital symbols.

14. The magnetic compass apparatus of claim 9, wherein said means for processing and said means for signal conditioning are one or more processors resident on one or more integrated circuit chips.

15. The magnetic compass apparatus of claim 9, wherein said means for processing and said means for signal conditioning comprise at least one software program.

16. The magnetic compass apparatus of claim 9, wherein said magnetometer sensors and said means for gravity field strength sensing may be sensing continuously.