Method and device for magnetic field measurement by magnetometers

Abstract

Magnetic field measurement by a set of magnetometers which are linked to a same moveable support and which have different orientations of eigendirections with respect to this support A processor determines, during a measurement in a given position and orientation of the support and of the set of magnetometers, for each magnetometer, of the orientation deviation between the eigendirections of the magnetometer and a candidate magnetic field, the magnetometer for which this deviation is minimal, the magnetic field measured by this sensor being selected as the magnetic field measured by the set of magnetometers. The magnetometers are distributed in space in order to cover a maximum of different orientations.

Claims

1. A method comprising: measuring candidate magnetic fields using different magnetometers included in a moving device, wherein each of the different magnetometers have different eigendirections, computing a set of deviations respectively associated with the different magnetometers, wherein each of the deviations is computed as follows: $e^{j} = \min_{i \in 〚 1. . n 〛} (\overset{.fwdarw.}{d_{l}^{J}}, \overset{.fwdarw.}{B_{B}^{J}})$ wherein e.sup.j is a respective one of the deviations associated with a j-th magnetometer among the different magnetometers, ({right arrow over (d.sub.i.sup.J)}).sub.i∈[[1 . . . n]] is a vector representing an i-th eigendirection of the j-th magnetometer, {right arrow over (B.sub.B.sup.J)} is a vector representing the candidate magnetic field measured by the j-th magnetometer, ({right arrow over (d.sub.i.sup.J)},{right arrow over (B.sub.B.sup.J)}) is an angle between {right arrow over (d.sub.i.sup.J)}, and {right arrow over (B.sub.B.sup.J)}, n is an integer greater than 1, selecting a magnetometer among the different magnetometers, wherein the selected magnetometer is associated with the respective one of the deviations that is minimal and wherein the selected magnetometer has measured a magnetic field least impacted by non-linearities, and using by a magneto-inertial navigation system the magnetic field measured by the selected magnetometer wherein said magnetic field is the least impacted by non-linearities without having resorted to a specific calibration to treat said non-linearities.

2. The method according to claim 1, in which the candidate magnetic field for a respective one of the different magnetometers is measured by said respective one of the different magnetometers.

3. The method according to claim 1, in which the different eigendirections of a magnetometer are determined by modelling prior to said measuring.

4. A non-transitory computer program product including code instructions for the execution of the method according to claim 1, when said programme is executed on a computer.

5. A storage device readable by a computer equipment on which a computer program product includes code instructions for the execution of the method according to claim 1.

6. The method according to claim 1, comprising: calculating magnetic navigation information of the moving device using the selected magnetic field.

7. The method according to claim 6, wherein the magnetic navigation information comprises at least one of: a linear velocity, an angular velocity, a position, and a heading.

8. A device comprising magnetometers each having different eigendirections, the magnetometers being configured to measure candidate magnetic fields, wherein the device further comprises a processor that is configured to: compute the deviations respectively associated with the magnetometers, wherein each of the deviations is computed as follows: $e^{j} = \min_{i \in 〚 1. . n 〛} (\overset{.fwdarw.}{d_{l}^{J}}, \overset{.fwdarw.}{B_{B}^{J}})$ wherein e.sup.j is a respective one of the deviations associated with a j-th magnetometer among the magnetometers, ({right arrow over (d.sub.i.sup.J)}).sub.i∈[[1 . . . n]] is a vector representing an i-th eigendirection of the j-th magnetometer, {right arrow over (B.sub.B.sup.J)} is a vector representing the candidate magnetic field measured by the j-th magnetometer, ({right arrow over (d.sub.i.sup.J)},{right arrow over (B.sub.B.sup.J)}) is an angle between {right arrow over (d.sub.i.sup.J)}, and {right arrow over (B.sub.B.sup.J)}; n is an integer greater than 1, select a magnetometer among the magnetometers, wherein the selected magnetometer is associated with the respective one of the deviations that is minimal and wherein the selected magnetometer corresponds to the magnetometer having measured a magnetic field least impacted by non-linearities, and use by a magneto-inertial navigation system the magnetic field measured by the selected magnetometer wherein said magnetic field is the least impacted by non-linearities without having resorted to a specific calibration to treat said non-linearities.

9. The device according to claim 8, wherein the magnetometers comprise a flat configuration of the magnetometers with two eigenaxes, the magnetometers being parallel, orientations of the different eigendirections of the magnetometers being distributed such that eigenaxes of the magnetometers are separated angularly by an angle of 90°/m, wherein m is a number of the magnetometers.

10. The device according to claim 8, in which the magnetometers are parallel.

11. The device according to claim 8, in which the magnetometers are of triaxial type, the different eigendirections of the magnetometers being distributed in order to optimize a distribution of the different eigendirections of the magnetometers in space.

12. A navigation system comprising at least one device according to claim 8.

Description

DESCRIPTION OF THE FIGURES

(1) Other characteristics and advantages of the invention will become clearer from the description that follows, which is purely illustrative and non-limiting, and should be read with regard to the appended figures in which:

(2) FIG. 1 is a diagram of equipment for the implementation of the method according to the invention;

(3) FIG. 2 represents in greater detail an example of case for the implementation of the method according to the invention;

(4) FIG. 3a illustrates a configuration of a device in accordance with an embodiment of the invention in which two magnetometers are distributed in a same plane;

(5) FIG. 3b illustrates a flat configuration with three magnetometers;

(6) FIG. 4 illustrates a configuration with four magnetometers distributed in space.

DESCRIPTION OF ONE OR MORE EMBODIMENTS

(7) General Remarks

(8) With reference to FIG. 1, the proposed measurement device is for example used for the estimation of the movement of an object 1 moving in an ambient magnetic field (typically the Earth's magnetic field, which could be slightly altered by nearby metal objects), noted {right arrow over (B)}. As is known, the magnetic field is a three-dimensional vector field, that is to say associating a vector of three dimensions with each three-dimensional point in which the object is moveable.

(9) This object 1 may be any moveable object of which knowledge of the position is desired, for example a wheeled vehicle, a drone, etc., but also a pedestrian.

(10) The object 1 comprises in a case 2 (support) a plurality of magnetic measurement sensors 20, i.e. axial magnetometers 20. Axial magnetometer is taken to mean an element capable of measuring a component of said magnetic field, i.e. the projection of said magnetic field vector {right arrow over (B)} at the level of said magnetometer 20 along its axis.

(11) More precisely, the magnetometers 20 are integral with the case 2. They have a movement substantially identical to the case 2 and to the object 1 in the terrestrial reference frame.

(12) Preferably, the reference frame of the object 1 is provided with an orthonormal cartesian point of reference in which the magnetometers 20 have a predetermined position in this point of reference.

(13) In FIG. 2, the case 2 is fixed onto the object 1 (for example a limb of the pedestrian) by attachment means 23. These attachment means 23 consist for example of a bracelet, for example a self-gripping strap that grips the limb and enables an integral link.

(14) Obviously, the invention is not limited to the estimation of the movement of a pedestrian, but it is particularly advantageous in such a use because it enables very reduced bulk, which is necessary for the case to be portable by a human in an ergonomic manner.

(15) The case 2 may include processing means 21 (typically a processor) for implementing directly in real time the processing operations of the present method, or instead the measurements may be transmitted via communication means 25 to an external device such as a mobile terminal (smartphone) 3, or even a remote server 4, or instead the measurements may be recorded in local data storage memory means 22 (a flash type memory for example) for a posteriori processing for example on the server 4.

(16) The communication means 25 may implement a short range wireless communication, for example Bluetooth or Wi-Fi (in particular in an embodiment with a mobile terminal 3) or even be means for connecting to a mobile network (typically UMTS/LTE) for a long distance communication. It should be noted that the communication means 25 may be for example a wired connection (typically USB) for transferring data from the local data storage means 22 to those of a mobile terminal 3 or a server 4.

(17) If it is a mobile terminal 3 (respectively a server 4) that hosts the “intelligence”, it includes processing means 31 (respectively 41) such as a processor for implementing the processing operations of the present method that are going to be described. When the processing means used are those 21 of the case 2, it may further include communication means 25 for transmitting the estimated position. For example, the position of the bearer may be sent to the mobile terminal 3 to display the position in a navigation software interface.

(18) The data processing means 21, 31, 41 respectively of the case 2, a smartphone 3 and a remote server 4 may indifferently and depending on the applications carry out all or parts of the steps of the method.

(19) They each comprise to this end storage means in which are memorised all or part of the sequences of code instructions for the execution of the method.

(20) Arrangement and Orientation of the Magnetometers in Space

(21) Each magnetometer sensor, whatever its structure, has directions, called eigendirections in the remainder of the text, for which the impact of linearities is minimal.

(22) The magnetometers 20 are arranged in the case 2 so as to cover a multitude of different orientations for these eigendirections.

(23) In the case illustrated in FIGS. 3a and 3b, the case of biaxial magnetometers 20 each having two eigenaxes X and Y (in this particular instance perpendicular) has been represented.

(24) The planes formed by the eigenaxes X and Y of each of these magnetometers 20 are parallel (coplanar in the examples of these figures).

(25) In FIG. 3a, the sensor comprises two coplanar biaxial magnetometers 20. These magnetometers 20 are distributed in such a way that their eigenaxes are angularly separated by 45°.

(26) Indeed, it will be understood that such a distribution optimises the probability that a magnetic field in the plane XY is angularly close to the eigendirection of one of the sensors.

(27) In the case illustrated in FIG. 3b, the system comprises three biaxial magnetometers distributed according to a coplanar arrangement of the different eigenaxes X and Y of the magnetometers 20. These magnetometers 20 are oriented in such a way that the eigenaxes of two successive magnetometers are angularly separated by 30°.

(28) It will be understood again here that such a distribution optimises the probability that a magnetic field in the plane XY is angularly close to the eigendirection of one of the sensors.

(29) Obviously these two examples are generalised to the case of m coplanar biaxial magnetometers or distributed in parallel planes of the device (with m a whole number).

(30) In this case, the optimal distribution is a distribution of magnetometers separated by angles of 90°/m, but it will be easily understood that the invention is not limited to this optimal distribution.

(31) FIG. 4 illustrates for its part the case of four triaxial magnetometers each having three perpendicular eigenaxes X, Y and Z. It illustrates a distribution of the orientations of these four magnetometers in space which enables an optimisation of the angular proximity between a magnetic field B to measure and at least one eigendirection of one of the magnetometers 20.

(32) In a more general manner, in the case of triaxial magnetometers, the distribution of their orientations is chosen with a maximum of orientations in space in order to maximise the probability of finding a magnetometer that has the most optimal orientation (optimisation of the distribution of their eigendirections).

(33) Choice of the Magnetometer for a Given Measurement

(34) For each magnetometer j, ({right arrow over (d.sub.t.sup.j)}).sub.i∈[[1 . . . n]] designates the n measurement eigendirections of this sensor, that is to say the directions linked to this sensor for which the impact of non-linearities is minimal. The number of these eigendirections depends on the type of sensor.

(35) The processing means calculate for each magnetometer a candidate magnetic field by considering that the magnetometers are not disturbed by non-linearities. Indeed, since the effect of non-linearities is small, it is possible to assume as a first approximation that the magnetic field supplied by each sensor is not too poor, in any case that it suffices for the remainder of the method. The candidate magnetic field of a sensor may be the magnetic field measured by this sensor or an astute combination (such as the average for example) of the magnetic fields measured by all the sensors. Hereafter, the simplest case will be considered where the candidate magnetic field of a sensor is the magnetic field measured by said sensor.

(36) The processing means (depending on the case 21, 31 or 41) calculate for each sensor the orientation deviation between the magnetic field measured by the sensor and its eigendirections. If {right arrow over (B.sub.B.sup.J)} is the “candidate” magnetic field for the sensor j, the deviation e.sup.j between the eigendirections and {right arrow over (B.sub.B.sup.J)} is given by

(37) $e^{j} = \min_{i \in 〚 1. . n 〛} (\overset{.fwdarw.}{d_{l}^{J}}, \overset{.fwdarw.}{B_{B}^{J}})$

(38) where ({right arrow over (d.sub.i.sup.J)},{right arrow over (B.sub.B.sup.J)}) is the angle between the two vectors {right arrow over (d.sub.i.sup.J)} and {right arrow over (B.sub.B.sup.J)}.

(39) The processing means 21, 31 or 41 next determine the sensor j for which the deviation e.sup.j is minimal.

(40) It is from this sensor that the best measurement is obtained, in the sense where the impact of non-linearities is the lowest on the measurement.

(41) The magnetic field measured by the magnetometer j thereby determined is thus retained by the processing means 21, 31, 41 as the magnetic field measured by the set of magnetometers.

(42) The field measurement of the magnetometer j is stored by said processing means 21, 31, 41 and/or used by said processing means in the calculation of magneto-inertial navigation information (linear velocity, angular velocity, position, heading, etc.).

(43) In these calculations, the measurement of the field j is then used as measurement of the magnetic field at the considered time, corresponding to a position/orientation of the set of magnetometers.

(44) The processing operations are then advantageously of the type of those described in the patent FR2914739 or in the application FR 1653493.

(45) As will have been understood, the fact of using the “candidate” magnetic field of the magnetometer for which the deviation is minimal makes it possible to have available a measurement not disturbed by non-linearities of the magnetometers and to do so without having to resort to a specific calibration to treat said non-linearities.

(46) Obviously, the proposed calibration method and device may be used in combination with calibrations intended to correct not non-linearity errors but other error parameters of the sensor according to known methods.

(47) Eigendirections

(48) The eigendirections are determined beforehand for each sensor.

(49) This determination may be done for example from modelling of the error terms of the considered sensor, or by an analysis of the physics of the sensor and/or its implementation.

(50) An example of modelling and determination of eigendirections is given hereafter, other modellings and determinations obviously being possible.

(51) Case of AMR Magnetometers—Modelling of Non-Linearities

(52) The magnetometers 20 are for example each composed of three identical chips, the operation of which is based on so-called AMR (Anisotropic Magneto-Resistance) technology.

(53) When these chips are immersed in a magnetic field, they send back a voltage which comes close to an affine function of the component of the magnetic field in the direction of their sensitivity axis.

(54) Conventionally, the magnetometric measurements of a sensor of sensitivity axis X and of main plane XY (with Y the direction perpendicular to X) are modelled as follows (see for example Handbook of Magnetic Measurements, S. Tumanski):

(55) $M_{B}^{X} = \frac{M_{C}^{X}}{1 + M_{C}^{Y} / H_{0}}$

(56) Where M.sub.B.sup.X is the magnetic field measured by the sensor and where M.sub.C.sup.X and M.sub.C.sup.Y are the magnetic fields along the axes X and Y. H.sub.0 is the remanent field, assumed large compared to the magnetic fields in play.

(57) It is possible to form a triaxial AMR magnetometer. Such a magnetometer is analysed as formed of 3 monoaxial sensors, of which the non-linearity model is

(58) ${\begin{matrix} M_{B}^{X} = \frac{M_{C}^{X}}{1 + M_{C}^{Y} / H_{0}} \\ M_{B}^{Y} = \frac{M_{C}^{Y}}{1 - M_{C}^{X} / H_{0}} \\ M_{B}^{Z} = \frac{M_{C}^{Z}}{1 + M_{C}^{Y} / H_{0}} \end{matrix}$

(59) Case of AMR Magnetometers—Determination of Eigendirections

(60) As a function of the measured magnetic field, the impact of non-linearity is not the same on the measurement of the sensor. Thus, it may be directly read on the model that:

(61) When the field is aligned with the sensor axes, non-linearities do not hinder the measurement of the sensor. Indeed:

(62) If the field M.sub.C.sup.X is more or less aligned with the sensor axis X, one has M.sub.C.sup.Y≈M.sub.C.sup.X≈0, which gives
M.sub.B.sup.X≈M.sub.C.sup.X
M.sub.B.sup.Y≈0
M.sub.B.sup.Z≈0

(63) If the field M.sub.C.sup.Y is more or less aligned with the sensor axis Y, one has M.sub.C.sup.X≈M.sub.C.sup.Z≈0, which gives
M.sub.B.sup.X≈0
M.sub.B.sup.Y≈M.sub.C.sup.Y
M.sub.B.sup.Z≈0

(64) If the field M.sub.C.sup.Z is more or less aligned with the sensor axis Z, one has M.sub.C.sup.Z≈M.sub.C.sup.Y≈0, which gives
M.sub.B.sup.X≈0
M.sub.B.sup.Y≈0
M.sub.B.sup.Z≈M.sub.C.sup.Z

(65) When the field is not aligned with the sensor axes, non-linearities hinder the measurement of the sensor.

(66) In the case of a magnetic field oriented at 45° with respect to the axes X and Y, one then has

(67) $M_{C}^{X} \approx \frac{\sqrt{2}}{2} M_{C}$ $M_{C}^{Y} \approx \frac{\sqrt{2}}{2} M_{C}$ $M_{C}^{Z} \approx 0$

(68) Which gives a measured field

(69) ${\begin{matrix} M_{B}^{X} = \frac{\frac{\sqrt{2}}{2} M_{C}}{1 + \frac{\sqrt{2}}{2} M_{C} / H_{0}} \approx \frac{\sqrt{2}}{2} M_{C} - \frac{1}{2} \frac{M_{C}^{2}}{H_{0}} \\ M_{B}^{Y} = \frac{\frac{\sqrt{2}}{2} M_{C}}{1 - \frac{\sqrt{2}}{2} M_{C} / H_{0}} \approx \frac{\sqrt{2}}{2} M_{C} + \frac{1}{2} \frac{M_{C}^{2}}{H_{0}} \\ M_{B}^{Z} \approx 0 \end{matrix}$

(70) The optimal directions (eigenaxes) in the case of this model are thus the sensor axes.

Method and device for magnetic field measurement by magnetometers

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G01R33/0206

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G01V3/10

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G01C21/08

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G01R33/02

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Abstract

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Description