Method of compensation of electromagnetic perturbations due to moving magnetic and conducting objects

• US 5,694,041 A
• Filed: 06/28/1995
• Issued: 12/02/1997
• Est. Priority Date: 07/05/1994
• Status: Expired due to Term

• Current Assignee: Sextant Avionique
• Original Assignee: Sextant Avionique
• Inventors: Lescourret, Jean-Louis
• Primary Examiner(s): Snow, Walter E.
• Application Number:
  US08/496,213
• Time in Patent Office: 888 Days
• Field of Search: 324/207.12, 324/244, 324/260, 324/225, 324/247, 364/571.01, 364/571.02, 364/571.03, 364/571.04, 364/571.05, 364/571.06, 364/571.07, 364/571.08
• US Class Current: 324/207.12
• CPC Class Codes: G01R 33/025 Compensating stray fields G01R33/0017 takes precedence
• CPC Class Codes: G02B 27/017 Head mounted

Abstract

The disclosure relates to a method of compensation of perturbations of magnetic field measurements made by a sensor (C) caused by sources of perturbation (S₁, S₂, S₃, . . . S_k) attached to the sensor. The method utilizes a first stage that expresses the perturbation caused by each of the sources, that are situated in any given ambient magnetic field, as an explicit function of the average magnetic field at the position of the source so as to build a model that can be used to calculate its perturbation at the center of the sensor, a second stage, said sensor being located in the magnetic field to be measured, to determine the perturbation caused by each of the sources of perturbation by using its model and knowing the average magnetic field at this position, a third stage that sums the perturbations of all the sources of perturbation, and a fourth stage that subtracts this sum from the measurement made by the sensor. The invention is applicable to systems requiring precise measurement of a magnetic field, notably in the case of the determination of the position and orientation of aircraft pilot helmet aiming devices.

Claims

1. Method of compensation of perturbations of magnetic field measurements made by a sensor (C) caused by sources of perturbation (S₁,S₂,S₃, . . . S_k) attached to the sensor, wherein it comprises a first stage that expresses the perturbation caused by each of said sources, that are situated in a given ambient magnetic field, as an explicit function of the average magnetic field at the position of the source so as to build a model that can be used to calculate its perturbation at the center of said sensor, a second stage, said sensor being located in a magnetic field to be measured, to determine the perturbation caused by each of said sources of perturbation by using its model and knowing the average magnetic field at this position, a third stage that sums the perturbations of all said sources of perturbation, and a fourth stage that subtracts this sum from the measurements made by said sensor.

2. Method according to claim 1, wherein each source (S_k) is modelled by at least one loop (S_pK), the variations of the ambient field (B_k) at the position of the loop creating a current (i_k) in the loop, which produces a perturbing field.

3. Method according to claim 2, wherein the perturbation caused by each source (S_k) is modelled by a vector magnetic field (B_Pk (O_c)) at the position of said sensor (C), this field being expressed as follows for each component:

##EQU22## where;

(B_Pk (O_C)).sbsb.i is the ith component of the vector ##EQU23## at the position of the sensor (C) and produced by at least one modelling loop of the source (S_k), i=1, 2, 3 . . . ;

(B_k (O_k)) is the value of the average ambient magnetic field at the center O_k of the source (S_k) and |B_k (O_k)| is its modulus;

θ

_k is the angle between an axis ##EQU24## of the frame of reference of the sensor (C) and the vector magnetic field B_k (O_k) at the center O_k of the loop;

φ

_k is the angle between an axis ##EQU25## of the frame of reference of the sensor and the projection of the previously mentioned vector magnetic field B_k (O_k) on the plane passing through the center O_c of the frame of reference of the sensor and containing the axes ##EQU26## and ##EQU27## (O_c,X_c,Y_c,Z_c) being the frame of reference of the ) sensor;

P_l^m (x) is a Legendre polynomial of order l,m defined by;

##EQU28## A_l,m,i^k and C_l,m,i^k are constants.

4. Method according to claim 3, wherein the source (S_k) is modelled by three loops.

5. Method according to claim 3, wherein in order to determine the constants A_l,m,i^k and C_l,m,i^k and the coordinates, (X'"'"'_k)₁,2,3 of the source (k), the sensor (C) and the sources (S_k) are displaced by translation and rotation in a known ambient field, the constants A_l,m,k^k, C_l,m,i^k and (X'"'"'_k)₁,2,3 being identified by an iterative method in order that these constants minimize the mean quadratic deviation between the compensated measurement and the known field.

6. Method according to claim 1, applicable to the determination of the position and orientation of an aircraft pilot'"'"'s helmet aiming device, a transmitter fixed relative to the cockpit of the aircraft being composed of three coils each generating a magnetic field, the sensor (C), attached to the helmet, being composed of the same number of coils, the measurements by the sensor of the field produced by the transmitter enabling the position and orientation of the sensor to be defined in the space of the cockpit, the perturbations caused by the sources (S₁,S₂,S₃, . . . S_k) mounted on the helmet being compensated.

7. Method according to claim 6, wherein the constants A_l,m,i^k and C_l,m,i^k are determined independently for the field produced by each of said coils of the transmitter.

8. Method according to claim 5, wherein the constants A_l,m,k^k, C_l,m,i^k and (X'"'"'_k).sbsb.1,2,3 of the model of the sources of perturbation (S₁,S₂,S₃, . . . ,S_k) attached to the helmet are determined in the factory, the helmet being placed in any given field.

9. Method according to claim 5, wherein the constants A_l,m,i^k, C_l,m,i^k and (X'"'"'_k).sbsb.1,2,3 of the model are memorized in memorization circuits fitted in the helmet.