TECHNICAL FIELD

The invention relates to a method and to a device for estimating an angular deviation between the magnetic axis of the magnetic moment of a magnetic object and a reference axis of said magnetic object.

PRIOR ART

The use of magnetic objects is known, particularly within the context of a system for recording the trace of a magnetic pencil on a writing medium. The magnetic object in this case is understood to be an object with which a non-zero magnetic moment is associated, for example, a permanent magnet attached to a non-magnetic pencil.

By way of an example, document WO 2014/053526 discloses a system for recording the trace of a pencil to which an annular permanent magnet is attached. The magnetic object, in this case the permanent magnet, comprises a magnetic material, for example a ferromagnetic or ferrimagnetic material, evenly distributed around a mechanical axis, called reference axis, which corresponds to its axis of rotation. The magnet is designed so that its magnetic moment is substantially co-linear to the reference axis.

The system for recording the trace of the pencil provided with the permanent magnet comprises an array of magnetometers capable of measuring the magnetic field generated by the permanent magnet. The magnetometers are attached to a writing medium.

However, the trace recording method assumes that the magnetic axis of the permanent magnet, which is defined as the axis passing through the magnetic moment, is effectively co-linear to the reference axis, or exhibits an acceptable angular deviation between the magnetic axis and the reference axis. Indeed, an angular deviation of several tens of degrees can result in an error in recording the trace, which can then exhibit a detrimental lack of precision. It then may be necessary for a prior estimate to be made of the angular deviation between the magnetic axis and the reference axis of the magnetic object.

DISCLOSURE OF THE INVENTION

The aim of the invention is to propose a method for estimating an angular deviation between a reference axis of a magnetic object and a magnetic axis co-linear to a magnetic moment of said magnetic object. To this end, the estimation method comprises the following steps:a) positioning said magnetic object facing at least one magnetometer capable of measuring a magnetic field in the presence of the magnetic object;b) rotating said magnetic object about said reference axis;c) measuring, during the rotation and using said magnetometer, the magnetic field at different instants of a measurement duration;d) estimating the angular deviation from the measurements of the magnetic field.

Preferably, the estimation step comprises:a sub-step of identifying a magnetic field, called minimum magnetic field, and a magnetic field, called maximum magnetic field, from the measurements of the magnetic field; anda sub-step of computing the angular deviation from the identified minimum and maximum magnetic fields, and from geometric parameters representing the position of the magnetometer relative to the magnetic object.

Preferably, during the identification sub-step, the minimum and maximum magnetic fields are respectively identified from the minimum and maximum values of the norm of the measurements of the magnetic field.

Preferably, said geometric parameters are the coordinates and the distance of the magnetometer relative to the magnetic object, in a plane passing through the reference axis and containing the magnetometer.

The angular deviation can be computed from a coefficient equal to the ratio of the norm of the vector formed from the subtraction of the minimum and maximum magnetic fields to the norm of the vector formed from the addition of the minimum and maximum magnetic fields, and from said geometric parameters.

The angular deviation can be computed from the following equation:

α_=tan-1((Bmaxd-BmindBmaxd+Bmind)·(r2⁢z2+(z2-d2/a)2r2⁢z2+(r2-d2/a)2)),
where d is the distance between the magnetometer and the magnetic object, z and r are the coordinates of the magnetometer relative to the magnetic object along an axis, respectively parallel and orthogonal to the reference axis, and where a is a predetermined coefficient.

During the rotation step, the magnetic object can complete at least one rotation about the reference axis.

Said at least one magnetometer comprises at least three axes for detecting the magnetic field, said detection axes being non-parallel to each other.

Preferably, said at least one magnetometer is a single tri-axis magnetometer.

Preferably, said at least one magnetometer is positioned outside the reference axis or outside the perpendicular to the reference axis passing through the magnetic object.

Said at least one magnetometer can be positioned relative to said magnetic object at a coordinate z along an axis parallel to the reference axis and at a coordinate r along an axis orthogonal to the reference axis, such that the coordinate z is greater than or equal to the coordinate r.

The invention also relates to a method for characterizing a magnetic object having an angular deviation between a reference axis of said magnetic object and a magnetic axis co-linear to a magnetic moment of said magnetic object, comprising the following steps:implementing the method for estimating the angular deviation according to any of the preceding features;computing an amplitude of the magnetic moment associated with the magnetic object, from the estimated angular deviation, of the identified maximum magnetic field, and from said geometric parameters.

Preferably, the amplitude of the magnetic moment is computed from the ratio between the norm of the maximum magnetic field and the norm of a magnetic field, for a unitary amplitude of said magnetic moment, expressed analytically using the following equation:

B^maxd(α_,r,z)=bd5⁢(-sin⁡(α_)⁢d2a+sin⁡(α_)⁢r2+cos⁡(α_)⁢rz-cos⁡(α_)⁢d2a+cos⁡(α_)⁢z2+sin⁡(α_)⁢rz),
whereαis the previously estimated angular deviation, d is the distance between the magnetometer and the magnetic object, z and r are the coordinates of the magnetometer relative to the magnetic object along an axis, respectively parallel and orthogonal to the reference axis, and where a and b are predetermined coefficients.

According to another embodiment, the estimation method comprises the following steps:a) positioning said magnetic object facing a measurement plane defined by an array of magnetometers capable of measuring a magnetic field in the presence of the magnetic object;b) rotating said magnetic object about said reference axis;c) measuring, during the rotation and using said magnetometers, the magnetic field at different instants of a measurement duration;d) estimating a magnetic moment, called instantaneous magnetic moment, of the magnetic object, at said different measurement instants, from the measured magnetic field;e) estimating the mean angular deviation from said instantaneous magnetic moments.

The step of estimating the mean angular deviation can comprise a sub-step of estimating an invariant vector rotating about the reference axis from the instantaneous magnetic moments, the estimation of the mean angular deviation also being performed from said invariant vector.

The estimation of the mean angular deviation can comprise computing an angular deviation amplitude between the instantaneous magnetic moments relative to the invariant vector.

During the step of measuring the magnetic field, the magnetic object can complete a whole number, greater than or equal to 1, of rotations about the reference axis.

The estimation method can comprise a step of homogenizing an angular distribution of the instantaneous magnetic moments about the reference axis throughout the measurement duration, with this homogenization step comprising a computation by interpolating a time series, called homogenized series, of instantaneous magnetic moments, from a time series, called initial series, of previously obtained instantaneous magnetic moments, so that the successive instantaneous magnetic moments of the homogenized time series exhibit a substantially constant angular deviation.

The homogenization step can comprise an iterative phase of computing interpolated magnetic moments, during which a magnetic moment interpolated at a considered iteration is obtained from a magnetic moment interpolated at a previous iteration, a predetermined threshold value and a unit vector defined from two successive instantaneous magnetic moments of the initial time series.

The positioning step can comprise:a sub-step of estimating a first dispersion parameter representing a dispersion of the intensity of the instantaneous magnetic moments of the magnetic object, for various positions, called vertical positions, of the magnetic object along an axis substantially orthogonal to the measurement plane;a sub-step of positioning the magnetic object in a vertical position, for which the value of the first dispersion parameter is less than or equal to a predetermined threshold value.

The estimation method can comprise, prior to the positioning step, a step of measuring, using said magnetometers, an ambient magnetic field in the absence of the magnetic object, and can comprise a sub-step of subtracting the ambient magnetic field from the previously measured magnetic fields, so as to obtain the magnetic field generated by the magnetic object.

The estimation method can comprise a step of applying a low-pass filter to the values of components of the previously estimated instantaneous magnetic moments.

The low-pass filter can be a running mean on a given number of samples.

The number of samples can be predetermined so that a bias, which is defined as a difference between an angular deviation, called actual angular deviation, and the estimated mean angular deviation, is minimal, by absolute value, for a value called mean angular deviation threshold value.

The estimation method can comprise a step of sorting the magnetic object according to the difference in the estimated value of the mean angular deviation relative to a value called mean angular deviation threshold value.

The estimation method can comprise a step of estimating a parameter, called quality indicator, from the computation of a second dispersion parameter representing a dispersion of the values of a parameter, called instantaneous radius, computed as being the norm of an instantaneous vector defined between each instantaneous magnetic moment and the previously estimated invariant vector.

The estimation method can further comprise a step of comparing the second dispersion parameter to a predetermined threshold value.

Predetermining the threshold value can comprise the following sub-steps:obtaining, Q times, with Q being a non-zero natural integer, a time distribution of said instantaneous radius, for the same magnetic object;estimating, for each time distribution, a mean value, called mean radius, defined as the time mean of the instantaneous radius, so as to obtain a distribution of Q estimated mean radii;estimating, from the distribution of the Q estimated mean radii, a mean value and a third dispersion parameter, called threshold value, representing a dispersion of the values of the estimated mean radii.

The invention also relates to a device for estimating a mean angular deviation between a magnetic axis and a reference axis of a magnetic object, comprising:an array of magnetometers capable of measuring, at each measurement instant of a duration, a magnetic field disrupted by the magnetic object along at least one measurement axis, the magnetometers being spatially distributed so as to define a measurement plane;a retention and rotation component capable of retaining the magnetic object in a position facing the measurement plane and of rotating the magnetic object along its reference axis;a processor configure d to implement the estimation method according to any of the preceding features.

The invention also relates to an information recording medium, comprising instructions for implementing the estimation method according to any of the preceding features, said instructions being able to be executed by a processor.

