Automatic calibration of magnetic compasses
First Claim
1. A method of calibrating a magnetic compass comprising sensor means operable to supply signals representing the magnitude of a sensed magnetic field comprising an earth magnetic field component and a magnetic field distortion component, and means responsive to said signals for providing a heading as the angular displacement of an alignment axis of the compass relative to magnetic north, the method comprising the steps of:
- as the compass is displaced angularly, storing measured values of the magnitude of the sensed magnetic field for different discrete orientations of the alignment axis of the compass;
if said measured values have been stored for more than a predetermined number, but not all, of said directions, interpolating, using estimated coordinates for said origin, values of the sensed magnetic field for directions for which measured values have not been accumulated to provide a complete set of values (Xi, Yi) corresponding to a complete rotation of the alignment axis, said set of values defining a substantially elliptical locus of a vector (ET) representing the sensed magnetic field;
deriving from said set of values new estimated coordinates for said origin and adjusting the stored measured values to take account of differences, if any, between the previous estimated origin coordinates and the new estimated origin coordinates; and
deriving said heading taking into account ellipticity of said locus due to said local magnetic field distortion.
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Accused Products
Abstract
In a magnetic compass of the kind in which a sensor derives coordinates of a sensed magnetic field, automatic correction of hard-iron and soft-iron distortions is achieved by storing a set of measured values of the magnitude of the sensed magnetic field for different discrete directions of the alignment axis of the compass relative to an estimated origin. If measured values have been accumulated for more than a predetermined number, but not all, possible directions, values are interpolated for the remaining directions to provide a set of measured and interpolated values defining a substantially elliptical locus of the sensed magnetic field vector (ET.) New coordinates for the estimated origin are derived and the measured values adjusted to take account of changes in the estimated origin. The heading is corrected for displacement of the origin due to hard-iron effect and for ellipticity of the locus due to soft-iron effects. Correction of soft-iron effect may be achieved by determining the phase rotation angle (Ωe) of the elliptical locus relative to the alignment axis of the compass; deriving the ratio (ER /EL) between a pair of counter-rotating vectors, one (EL) representing the earth magnetic field component and the other (ER) soft-iron distortion, their being the sensed magnetic field vector (ET) and their phase difference φR being twice the rotation angle; and deriving from the ratio (ER /EL), the phase difference (φR), and the estimated origin headings corrected for said magnetic field distortion.
68 Citations
48 Claims
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1. A method of calibrating a magnetic compass comprising sensor means operable to supply signals representing the magnitude of a sensed magnetic field comprising an earth magnetic field component and a magnetic field distortion component, and means responsive to said signals for providing a heading as the angular displacement of an alignment axis of the compass relative to magnetic north, the method comprising the steps of:
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as the compass is displaced angularly, storing measured values of the magnitude of the sensed magnetic field for different discrete orientations of the alignment axis of the compass; if said measured values have been stored for more than a predetermined number, but not all, of said directions, interpolating, using estimated coordinates for said origin, values of the sensed magnetic field for directions for which measured values have not been accumulated to provide a complete set of values (Xi, Yi) corresponding to a complete rotation of the alignment axis, said set of values defining a substantially elliptical locus of a vector (ET) representing the sensed magnetic field; deriving from said set of values new estimated coordinates for said origin and adjusting the stored measured values to take account of differences, if any, between the previous estimated origin coordinates and the new estimated origin coordinates; and deriving said heading taking into account ellipticity of said locus due to said local magnetic field distortion. - View Dependent Claims (3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35)
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2. A method as claimed in claim wherein each interpolated value is generated by performing a linear interpolation between values for directions adjacent the direction for which the interpolated value is to be generated.
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16. A method of calibrating a magnetic compass comprising a sensor means operable to supply signals representing the magnitude of a sensed magnetic field comprising an earth magnetic field component and a magnetic field distortion component, and means responsive to said signals for providing a heading as the angular displacement axis of the compass relative to magnetic north, the method comprising the steps of:
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deriving a set of values representing the direction and magnitude of a sensed magnetic field as the compass alignment axis is displaced angularly, said set of values defining a substantially elliptical locus of a full rotation of vector (ET) representing the sensed magnetic field; determining from said set of values the phase rotation angle (Ω
e) of said elliptical locus relative to said alignment axis and coordinates of the origin of the elliptical locus;deriving from said set of values the ratio between a pair of counter-rotating vectors, one of the pair of vectors (EL) representing the earth magnetic field component and the other of said pair of vectors (ER) representing soft-iron magnetic field distortion, the resultant of such pair of vectors comprising the sensed magnetic field vector (ET) and the phase, difference (φ
R) between the vectors of the pair being twice the phase rotation angle (Ω
e);deriving from said ratio (ER /EL), said phase rotation angle (φ
R), and said coordinates of the origin, headings corrected for said soft-iron magnetic field distortion. - View Dependent Claims (17, 18, 19, 20, 21, 22, 23, 24)
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25. A magnetic compass comprising sensor means (10-27) operable to supply signals representing the magnitude of a sensed magnetic field comprising an earth magnetic field component and a magnetic field distortion component, and means (28) responsive to said signals for providing a heading as the angular displacement of an alignment axis of the compass relative to magnetic north, the means responsive to said signals comprising storage means (37) for storing, as the compass is displaced angularly, measured values (XE,YE) of the magnitude of the sensed magnetic field for different discrete directions of the alignment axis of the compass relative to an origin of the sensor;
- interpolator means (39) operable if said measured values have been stored for more than a predetermined number, but not all, of said directions, to interpolate, using estimated coordinates for said origin, values of the magnetic field for directions for which measured values have not been accumulated to provide a complete set of values (Xi Yi) corresponding to a complete rotation of the alignment axis, said set of values defining a substantially elliptical locus of a vector (ET) representing the sensed magnetic field;
origin computation means (35) for deriving from said set of values new estimated coordinates for said origin and means (40), responsive to the set of values and the new estimated origin coordinates for adjusting the stored measured values to take account of differences, if any, between the previous estimated origin coordinates and the new estimated origin coordinates; and
means (36) responsive to said set of values and said new estimated origin coordinates for deriving said heading taking into account ellipticity of said locus due to said local magnetic field distortion. - View Dependent Claims (36, 37, 38, 39)
- interpolator means (39) operable if said measured values have been stored for more than a predetermined number, but not all, of said directions, to interpolate, using estimated coordinates for said origin, values of the magnetic field for directions for which measured values have not been accumulated to provide a complete set of values (Xi Yi) corresponding to a complete rotation of the alignment axis, said set of values defining a substantially elliptical locus of a vector (ET) representing the sensed magnetic field;
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40. A magnetic compass comprising sensor means (10-27) operable to supply signals (XE {k}, YE {k}) representing the magnitude of a sensed magnetic field comprising an earth magnetic field component and a magnetic field distortion component, and means (28) responsive to said signals for providing a heading as the angular displacement axis of the compass relative to magnetic north, the means responsive to said signals being functional to derive a set of values representing the direction and magnitude of a sensed magnetic field as the compass alignment axis is displaced angularly, said set of values defining a substantially elliptical locus of a full rotation of vector (ET) representing the sensed magnetic field;
- determine from said set of values the phase rotation angle (Ω
e) of said elliptical locus relative to said alignment axis and coordinates of the origin of the elliptical locus;
derive from said set of values the ratio between a pair of counter-rotating vectors, one of the pair of vectors (EL) representing the earth magnetic field component and the other of said pair of vectors (ER) representing non-permanent magnetic field distortion, the resultant of such pair of vectors comprising the sensed magnetic field vector (ET) and the phase difference (φ
R) between the vectors of the pair being twice the phase rotation angle (Ω
e); and
derive from said ratio (ER /EL), said phase rotation angle (φ
R), and said coordinates of the origin, headings corrected for said non-permanent magnetic field distortion. - View Dependent Claims (41, 42, 43, 44, 45, 46, 47, 48)
- determine from said set of values the phase rotation angle (Ω
Specification