Calibration of magnetic compasses

Claim 2

A magnetic compass for a vehicle adapted to enable the output of the compass to be compensated by disturbance to the magnetic field caused by said vehicle, and including (i) three magnetic sensors responsive to respective nominally orthogonal components of the disturbed magnetic field acting along respective nominally orthogonal body axes of the vehicle for producing signals corresponding to said components; (ii) data storage means for accumulating data output by said sensors during a setting manoeuvre of the vehicle; (iii) data processing means for processing data stored in said data storage means to fit a generalised reduced quadric equation having six or less coefficients and defining an ellipsoid representing the locus of values of said signals and for determining said coefficients; (iv) calibration means for deriving from said coefficients a calibration function for adjusting values output by said magnetic sensors so that said locus of adjusted values is a spheroid centred on the origin of the body axes, and (v) compensation means for receiving raw data from said magnetic sensors and for implementing said calibration function thereby to obtain signals corresponding to orthogonal components of the magnetic field in which said disturbance has been compensated.

Claim 3

A compass according to claim 1, wherein the said data processing means process data stored in said data storage means to fit a generalised reduced quadric equation of the form: where x,y and z are values representing the magnetic field in the x,y and z directions, and determine the coefficients a,b,h,p,q and s.

Claim 4

A compass according to claim 2, wherein the data from the magnetic sensors is stored during a setting routine at the end of which the stored data is employed to derive the coefficients a,b,h,p,q and s.

Claim 5

A compass according to claim 3, wherein the data processing means employ a least squares method to fit the x,y,z data to the 6 state quadric equation giving rise to an equation of the form where a is a 6 x 1 vector comprising the coefficients a,b,h,p,q and s; A is a 6 x 6 accumulation matrix array containing products of x,y and z data, and v is a 6 x 1 accunulation vector containing products of x,y and z data.

Claim 6

A magnetic compass for a vehicle adapted to enable the output of the compass to be compensated for disturbance to the magnetic field caused by said vehicle and for adjusting said compensation to allow for variation in said disturbance and including, (i) three magnetic sensors responsive to respective nominally orthogonal components of the disturbed magnetic field acting along respective orthogonal body axes of the vehicle for producing signals corresponding to said components; (ii) model coefficient storage means for storing initial values for the coefficients of a generalised quadric equation defining an ellipsoid representing the locus of values of signals x,y and z output by said magnetic sensors; (iii) adaptive processing means for receiving and processing continuously data from said sensors to update said coefficients to compensate for variations in said disturbance of the magnetic field; (iv) calibration means for deriving from said updated coefficients a.b,h,p,q and s a calibration function for adjusting the values of x,y,z output by said magnetic sensors so that said locus of values of signals x,y and z is transformed into a sphere centred on the origin of the body axes of the vehicle; (v) compensation means for receiving raw data from said magnetic sensors and for implementing said calibration function thereby to obtain signals corresponding to orthogonal components of the magnetic field in which said disturbance has been compensated.

Claim 7

A magnetic compass for a vehicle adapted to enable the output of the compass to be compensated for disturbance to the magnetic field caused by said vehicle and for adjusting said compensation to allow for variation in said disturbance and including, (i) three magnetic sensors responsive to respective nominally orthogonal components of the disturbed magnetic field acting along respective orthogonal body axes of the vehicle for producing signals corresponding to said components; (ii) model coefficient storage means for storing initial values for the coefficients of a generalised quadric equation defining an ellipsoid representing the locus of values of signals x,y and z output by said magnetic sensors; (iii) adaptive processing means for receiving and processing continuously data from said sensors to update said coefficients to compensate for variations in said disturbance of the magnetic field; (iv) calibration means for deriving from said updated coefficients a.b,h,p,q and s a calibration function for adjusting the values of x,y,z output by said magnetic sensors so that said locus of values of signals x,y and z is transformed into a sphere centred on the origin of the body axes of the vehicle; (v) compensation means for receiving raw data from said magnetic sensors and for implementing said calibration function thereby to obtain signals corresponding to orthogonal components of the magnetic field in which said disturbance has been compensated.

Claim 8

A magnetic compass for a vehicle adapted to enable the output of the compass to be compensated for disturbance to the magnetic field caused by said vehicle and for adjusting said compensation to allow for variation in said disturbance, and including (i) three magnetic sensors responsive to respective nominally orthogonal components of the disturbed magnetic field acting along respective body axes of the vehicle for producing signals corresponding to said components; (ii) accumulation data storage means for accumulating data output by said sensors; (iii) calibration data storage means for storing data representing a matrix relating a vector representing the coefficients to a vector representing the accumulation data contained in said accumulation data storage means, and (iv) signal processing means for processing data output by said magnetic sensors and for updating the data stored in said calibration data storage means in accordance therewith to compensate for variations in the disturbances to the magnetic field.

Claim 9

A magnetic compass for a vehicle adapted to enable the output of the compass to be compensated for disturbance to the magnetic field caused by said vehicle and for adjusting said compensation to allow for variation in said disturbance, and including (i) three magnetic sensors responsive to respective nominally orthogonal components of the disturbed magnetic field acting along respective body axes of the vehicle for producing signals corresponding to said components; (ii) accumulation data storage means for accumulating data output by said sensors; (iii) calibration data storage means for storing data representing a matrix relating a vector representing the coefficients to a vector representing the accumulation data contained in said accumulation data storage means, and (iv) signal processing means for processing data output by said magnetic sensors and for updating the data stored in said calibration data storage means in accordance therewith to compensate for variations in the disturbances to the magnetic field.

Claim 10

A compass according to claim 6, wherein variations in disturbance in the magnetic field can be compensated for, and the speed with which the adaptation is made can be controlled.