Automatic calibration of a three-axis magnetic compass

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.

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.