RU2572109C1 - Method to calibrate electronic magnetic compass - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: method to calibrate a magnetic compass consists in installation of the magnetic compass on the plane in four orthogonal positions and measurement of average values of magnetic field in each position, and also average values by all positions of the magnetic compass. Produced values make it possible to further analytically calculate the true azimuth of movement by results of measurements of the magnetic compass.

EFFECT: elimination of instrumental errors of a magnetic compass and increased accuracy of azimuth detection for object movement α in a plane.

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Description

The inventive method of calibration of an electronic magnetic compass (MK) relates to methods for constructing devices used on moving objects. The method can be used mainly for calibrating a pedestrian’s autonomous navigation system, for example, in order to increase the accuracy of determining the azimuth of an object’s movement according to MK data in the absence of signals from global navigation systems (GNS).

Earth's magnetic field can be used to solve navigation problems. However, this field is subject to the influence of numerous disturbing factors [1], including global and local anomalies, instrumental errors of the equipment, etc.

A significant contribution to the errors of the magnetometers is made by the instrumental errors of the device itself. The signal M coming from any magnetometer channel can be represented as M = kH + m, where k is the transmission coefficient of the channel, H is the actual magnetic field strength at the measurement point, m is the static error of the magnetometer (value of M in the absence of a magnetic field) . The problem is that for two-, three-channel magnetometers, the values of k and m along the orthogonal axes differ from each other. As a result, when measuring a constant magnetic field and arbitrary rotation of a triaxial magnetometer, an ellipsoid surface with a center not at the origin is obtained. For calibration (determination of unknown magnetometer parameters k and m), the obtained measurement results are approximated by an ellipsoid and the desired values are calculated using numerical methods, for example, the least squares method. This procedure is distinguished by high complexity. In many applications, satisfactory calibration accuracy can be achieved by breaking the three-dimensional calibration problem in 0XYZ coordinates into two two-dimensional problems: in the 0XY and 0YZ planes. In some cases, for example, when moving along the Earth’s surface, to determine the azimuth — the direction to the magnetic pole (MP), it is enough to perform a calibration in the Earth’s plane. However, even for a flat problem, the algorithms known to the authors for calculating the magnetometer parameters are quite complicated.

A known method of calibration [2] of any vector measuring instruments (magnetometers, accelerometers, antenna arrays, etc.), which consists in rotating the instruments at different angles and measuring the corresponding values with the calculation of the required adjustments.

The disadvantage of this method is its high complexity.

A known method of calibrating an electronic magnetic compass [3], which consists in moving it along a specific path and comparing its readings with GNS data.

The disadvantage of this method is the low accuracy of the calibration, due to the errors of the GNS determination of the path angle (azimuth).

A known method of calibrating an electronic magnetic compass [4], which consists in measuring magnetic fields with a large number of magnetometers and the formation of a distorting matrix of the calibrated electronic magnetic compass.

The disadvantage of this method is the high complexity of the calibration.

A known calibration technique for a biaxial electronic magnetic compass in information-measuring systems [5].

The disadvantage of this method is also the high complexity of the calculations.

Closest to the claimed is a method of calibrating an electronic magnetic compass [6], pp. 25-28, which consists in installing the compass on a selected plane, rotating the compass around an axis perpendicular to this plane, and fixing it in four, i = 1 ÷ 4, the orthogonal positions along the two orthogonal axes X and Y lying in the indicated plane, in each position the components of the magnetic field are received by the compass.

The disadvantage of this method is also the high complexity of the calculations.

The problem solved by the claimed invention is the creation of an easy to implement calibration method.

To solve this problem, in the method of calibrating the electronic magnetic compass, which consists in installing the compass on a plane, rotating the compass around an axis perpendicular to this plane and fixing it in four, i = 1 ÷ 4, orthogonal positions along two orthogonal axes X and Y, lying in the indicated plane, in each position the compasses take the components of the magnetic field, along the X and Y axes measure the average values of the magnetic field M _xi and N _yi in each, i = 1 ÷ 4, the position of the compass and the average values of the magnetic field for all positions compass m _x and m _y , calculate k - the degree of instrumental asymmetry of the transmission coefficients k _x and k _{y of} the compass receivers along the X and Y axes according to the formula:

when using a compass, the X axis is combined with the direction of motion and the true direction to the magnetic pole in the XY plane is calculated by the formula:

α = arctan [(B _y -m _y ) / k (B _x -m _x )],

where B _x and B _y are the components of the magnetic field measured by the compass along the X and Y axes.

Significant differences of the proposed method are that for calibrating the compass you need to measure only the average values of the magnetic field M _xi and M _yi in each, i = 1 ÷ 4, the position of the compass and the average values of the magnetic field for all positions of the compass m _x and m _y . The measurement results make it easy, using analytical expressions, to obtain the degree of instrumental asymmetry k of the transmission coefficients k _x and k _{y of} the compass receivers, and subsequently, also analytically, to find the direction to the magnetic pole.

In the prototype, compass calibration involves complex measurements and optimization calculations.

The inventive method is illustrated by the following graphic materials:

FIG. 1. Compass sensors.

The projections of the magnetic field vector on the axis 0X and 0Y when the compass rotates.

FIG. 2. Projections of the magnetic field vector during compass rotation.

FIG. 3. The scheme for determining the direction of the MP.

FIG. 4. The scheme of the device that implements the method, where:

1, 2 - field sensors;

3, 4, 5, 6 - integrators;

7 - computer.

Consider the possibility of implementing the proposed method for calibrating an electronic magnetic compass in solving the simplest problem: determining the azimuth of motion when moving along the Earth's surface.

The sensors of the triaxial electronic magnetic compass, FIG. 1 are arranged orthogonally in the 0XYZ basis, and the directions of the respective axes are displayed on the compass body. The magnetic field vector H is directed at the magnetic field. Each of the compass sensors receives signals M _x , M _y and M _z corresponding to the projections H _X , H _Y and H _{Z of the} vector H, however, each channel receives them with the transmission coefficient k peculiar to it and the static error m:

We assume that during calibration the magnitude and direction of the vector H do not change. This fact can be verified by repeatedly measuring and comparing the values of M _x , M _y and M _z . We will also assume that in the case under consideration the projection H _{Z is} not significant.

The compass is mounted on a plane parallel to the surface of the Earth on which the compass plane 0XY is mounted, FIG. 2a).

The average values of the magnetic field M _x1 and M _{y1 are} measured. In the simplest case, the average value of the magnetic field can be obtained as a result of one measurement. However, a valid estimate of the average value allows to a certain extent to get rid of fluctuations in the magnetic field, measurement errors, etc. To obtain the average value in the analog compass, the input signals M are integrated for a fixed time interval, and in the digital compass, a certain number of input samples are added up. As a result of the measurements, the averaged values are obtained:

Turn the compass 90 °, FIG. 2b), and the average values of the magnetic field components are measured in a similar manner:

The next turn of the compass, FIG. 2, c), will give the values:

Finally the fourth turn, FIG. 2d), will make it possible to obtain:

Simultaneously with measuring the average values of the magnetic field along the X and Y coordinates in each of the positions of the compass, the average coordinate values of the field for all positions of the compass are measured. Why integrate the compass coordinate readings obtained through the four stages of calibration, in the analog version or summarize in digital. Adding relations (2) - (5), it is easy to verify that the average values of the magnetic field for all positions of the compass are averaged static errors of the compass for each coordinate:

The obtained values allow us to calculate the ratio of the transmission coefficients of the compass on channels X and Y

those. the degree of asymmetry of the channels X and Y of the compass, and the values of the coefficients k _x and k _y themselves are not required.

Knowing the values of m _x , m _y and k allows using the compass to adjust its readings. If the X axis of the compass, FIG. 3, combine with the direction of motion (ND) of the object and measure the components of the magnetic field B _x and B _y , then the azimuth is the angle α between the X axis and the direction to the MP can be calculated by the formula:

If you need three-dimensional calibration, similar actions can be performed first in the 0XY plane, and then in the orthogonal plane, 0XZ or 0YZ.

A diagram of a device implementing the inventive method is shown in FIG. 4. The signals from the sensors 1 (2) of the magnetic field along the X (Y) axes arrive at integrators 3 and 4 (5 and 6), respectively. Integrator 3 (5) determines the average field value at each position of the compass, and integrator 4 (6) determines all four dimensions. The start and end of integration is determined by the control signals from computer 7. The latter saves the values of M _xi , M _yi , m _x and m _y , and also calculates by formula (6) k the degree of instrumental asymmetry of the transmission coefficients k _x and k _{y of} the compass receivers along the X axes and Y. After calibration, the compass is ready to go. When solving navigation problems, FIG. 3, the compass axis X is guided in the direction of travel, the magnetic field components B _x and B _y are measured, and computer 7 calculates the azimuth α by formula (7).

Thus, the inventive method allows the calibration of the electronic magnetic compass in one plane by simple means: both by technical implementation and by calculation algorithms, due to the obtained analytical relations.

Information sources:

1. _gps //

2. Patent WO 2013188776.

3. Patent RU 2503923.

4. Patent RU 2497139.

5. http://www.masters.donntu.edu.ua/2007/kita/gems/library/calibration.htm

6. Ivanov D.S., Tkachev S.S., Karpenko C.O., Ovchinnikov M.Yu. Calibration of sensors to determine the orientation of a small spacecraft // KIAM Preprints named after M.V. Keldysh. 2010. No. 28. 30 sec URL: http://library.keldysh.ru/preprint.asp?id=2010-28.

Claims (1)

The method of calibrating the electronic magnetic compass, which consists in installing the compass on a plane, rotating the compass around an axis perpendicular to this plane, and fixing it in four, i = 1 ÷ 4, orthogonal positions along two orthogonal axes X and Y lying in the specified planes, in each position the compass receives magnetic field components, characterized in that the X and Y axes measure the average values of the magnetic field M _xi and M _yi in each, i = 1 ÷ 4, the position of the compass and the average values of the magnetic field for all positions of the compass m _x and m _y calculate k - the degree of instrumental asymmetry of the transmission coefficients k _x and k _{y of} the compass receivers along the axes X and Y according to the formula:

when using a compass, the X axis is combined with the direction of motion and the true direction to the magnetic pole in the XY plane is calculated by the formula

where B _x and B _y are the components of the magnetic field measured by the compass along the X and Y axes.

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