Method and device for tracking an end of a utensil with recalibration of a distance between the end and a magnetic object securely attached to the utensil

Description

DOMAINE TECHNIQUE

[0001] The invention relates to the tracking, by a network of magnetometers, of the tip of a utensil equipped with a magnetic object, this utensil then being manipulated by a user. The invention finds an application in particular in the context of the recording of the trace made by the tip of a utensil on a reference surface, or even in the context of the human-machine interface and the manipulation of virtual objects by the user by means of the utensil. ÉTAT DE LA TECHNIQUE ANTÉRIEURE

[0002] Methods are known for tracking a utensil equipped with a magnetic object by means of a network of magnetometers, in particular in the context of recording the trace of the tip of a utensil on a reference surface, for example the tip of a stylus, a pencil or other, making it possible in particular to digitize the drawing or writing made by a user handling the utensil.

[0003] Document WO2013 / 144338A1 describes an example of a method for tracking the tip of a utensil using a network of magnetometers that tracks the position of a magnet attached to the utensil. The distance between the magnet and the tip is fixed and does not vary over time. A computing unit determines the position and orientation of the magnet, and then, knowing the fixed value of the magnet-tip distance, deduces the position of the tip.

[0004] However, during the handling of the utensil by a user, it may happen that this magnet is displaced from its initial position, the actual value of the magnet-tip distance then being different from the initial value set. This can lead to an error in determining the position of the tip. EXPOSÉ DE L'INVENTION

[0005] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose a method and a system for tracking a tip of a utensil by means of a network of magnetometers, which makes it possible to reduce the error in determining the position of the tip when the distance between the tip and a magnetic object secured to the utensil is modified during the handling of the utensil.

[0006] For this, the subject of the invention is a method for tracking a first end of a utensil handled by a user, a magnetic object being integral with the utensil and distant from the first end along a longitudinal axis Δ of the utensil by a first non-zero distance, the magnetic object having a magnetic moment m collinear to the longitudinal axis Δ, the method comprising the following steps: manipulation of the utensil by a user, the first end being alternately in contact and in non-contact with a reference surface, measurement of a magnetic field emitted by the magnetic object at different successive measurement times, by a network of magnetometers secured to the reference surface; determination of a state vector XA (tn ) representative of the position PA (tn ) and of the magnetic moment m(tn ) of the magnetic object, from the measurements of the magnetic field; determination of the position P p (tn ) of the first end of the utensil from the state vector XA (tn ) and a previously recorded value of the first distance between the first end and the magnetic object.

[0007] According to the invention, the first distance between the first end and the magnetic object can be modified during manipulation of the utensil while the first end is not in contact with the reference surface, by translation of the magnetic object along the longitudinal axis Δ so that it has a so-called effective value different from the recorded value. The method then comprises the following steps: determination of a parameter κ(tn ) representative of a contact between the first end and the reference surface belonging to a tactile sensor, and when the contact is detected by the tactile detector: o determination of a value of a first distance called projected, this being equal to a distance between the position P a (tn ) of the magnetic object and a position PI (tn ) of a point of intersection I between the reference surface and a straight line collinear with the magnetic moment m(tn), then o determination of the effective value of the first distance from the determined value of the first projected distance; o updating of the recorded value of the first distance by replacing the previously recorded value with the determined effective value of the first distance.

[0008] Some preferred but non-limiting aspects of this method are as follows.

[0009] The value of the first projected distance can be determined from a standard ∥P A P I (tn )∥ of a first vector P A P I (tn ) going from the position PA of the magnetic object to the position PI of the intersection point I, this norm ∥ P A P I (tn )∥ of the first vector P A P I (tn) being determined from a norm || m || of the magnetic moment m,from the coordinate mz of the magnetic moment along a vertical axis orthogonal to the plane of the reference surface, and from a first distance h A,z along the vertical axis between the position PA of the magnetic object and the reference surface.

[0010] When contact is detected, the position PI of the intersection point I can be determined from the position PA of the magnetic object and from the first vector P A P I (tn), this being determined from the standard ∥P A P I (tn )∥ previously determined, of the magnetic moment m and the standard || m || of the magnetic moment.

[0011] The first end may have a curved shape, and when contact is detected, the magnetic moment mmay have a non-zero inclination angle ϕ with respect to the vertical axis, so that the first end comprises an end zone located in the extension of the longitudinal axis Δ, as well as a contact zone, distinct from the end zone, at the level of which the first end is in contact with the reference surface, an effective contact position PC then being determined from the position PA and the magnetic moment m of the magnetic object, and from at least one pre-recorded geometric parameter representative of the curved shape of the first end.

[0012] The pre-recorded geometric parameter can be the center of a sphere of radius R whose surface is locally tangent to a surface of the first end (4) and passing through the position PP and through the effective contact position PC, the effective contact position PC can be determined from the magnetic moment mand its standard || m || and from a distance h' A,z along the vertical axis between the position PA of the magnetic object and a plane parallel to the reference surface and passing through the center of the sphere.

[0013] The effective PC contact position of the first end can be displayed on a graphical interface.

[0014] Can be displayed on a graphical interface, the PP position of the first end, or a position of the first end determined by projection of the PP position onto the reference surface along an axis collinear with the magnetic moment m or along a vertical axis orthogonal to the reference surface.

[0015] The PP position of the first end can be determined from the magnetic moment m , of its standard || m||, and the recorded value of the first projected distance, or when no update of the first distance has yet taken place, the pre-recorded value of said first distance.

[0016] The step of determining the first projected distance may not be when the magnetic moment m of the magnetic object has an inclination angle ϕ greater than 60° with respect to a vertical axis orthogonal to the plane of the reference surface.

[0017] The touch sensor can be inactive as long as the position PP of the first end is greater than a predefined threshold value PP,th along a vertical axis orthogonal to the plane of the reference surface, and can be electrically activated when the position PP is less than or equal to the threshold value PP,th.

[0018] The touch sensor may be a matrix sensor comprising a matrix of distinct pixels secured to the reference surface and adapted to provide an electrical response signal representative of a contact of the first end on the reference surface. The method may comprise a determination of at least one so-called closest pixel having a distance from the position PI less than or equal to a predefined threshold value, only the closest pixel being activated to detect the contact.

[0019] The utensil may comprise a second end opposite the first end along the longitudinal axis Δ, the magnetic object being distant from the second end along a longitudinal axis Δ by a second non-zero distance. The method may comprise a step of determining the orientation +m Or -m of the magnetic moment m directed towards the first end.

[0020] The tracking method may further track the second end of the utensil, wherein the second distance between the second end and the magnetic object may be changed during handling. The method comprises the following steps: during the step of manipulation of the utensil by a user, the second end is alternately in contact and in non-contact with a reference surface during the manipulation of the utensil while the second end is not in contact with the reference surface, the second distance is modified by translation of the magnetic object along the longitudinal axis Δ so that it has a second so-called effective value different from a second recorded value; determination of the position PP' (tn ) of the second end of the utensil from the state vector X a (tn ), the previously recorded value of the second distance between the second end and the magnetic object, and the orientation of the magnetic moment mtowards the second end; determination of a parameter κ'(tn ) representative of a contact between the second end and the reference surface, and when the contact is detected by the touch sensor: o determination of a value of a second distance called projected, this being equal to a distance between the position PA (tn ) of the magnetic object and a position PI (tn ) of a point of intersection I between the reference surface and a straight line collinear with the magnetic moment m (tn ), then; o determination of the effective value of the second distance from the determined value of the second projected distance; o updating of the recorded value of the second distance by replacing the previously recorded value with the determined effective value of the second distance.

[0021] The invention also relates to a system for tracking a first end of a utensil, comprising: a utensil, o intended to be manipulated by a user to bring the first end into alternate contact and non-contact with a reference surface; o a magnetic object being integral with the utensil and spaced from the first end along a longitudinal axis Δ of the utensil by a first non-zero distance which can vary during manipulation of the utensil by a user while the first end (4) is not in contact with the reference surface (2) by translation of the magnetic object along the longitudinal axis Δ so that it has a so-called effective value different from a recorded value, the magnetic object having a magnetic moment mcollinear to the longitudinal axis Δ; a location device, comprising: o a network of magnetometers, integral with the reference surface, and adapted to measure a magnetic field emitted by the magnetic object; o an electronic calculation unit, comprising a previously recorded value (D 1(k) ) of the first distance between the first end and the magnetic object, and being adapted to: ▪ determine a state vector XAXA (tn ) representative of a position PA (tn ) and of the magnetic moment m(tn ) of the magnetic object from the measured magnetic field; ▪ determining a position PP (tn ) of the first end from the state vector XA (tn ) and the previously recorded value (D 1(k) ) of the first distance between the first end and the magnetic object; ▪ when contact is detected between the first end and the reference surface by a touch sensor, determining a value of a first distance called projected, this being equal to a distance between the position PA (tn ) of the magnetic object and a position PI (tn ) of a point of intersection I between the reference surface and a straight line collinear with the magnetic moment m(tn), then determining the actual value of the first distance from the determined value of the first projected distance; and updating the recorded value of the first distance by replacing the previously recorded value with the determined actual value of the first distance; the touch sensor, comprising the reference surface, adapted to detect contact between the first end and the reference surface, and to transmit the contact information to the location device. BRÈVE DESCRIPTION DES DESSINS

[0022] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which: there figure 1 is a schematic and partial view, in section, of a system for tracking a tip of a utensil on a reference surface according to one embodiment; figure 2 is a schematic and partial exploded view of the tracking system illustrated in the fig.1 ; there figure 3 is a flowchart illustrating steps of a method for tracking the tip of a utensil according to one embodiment; figure 4A is a schematic and partial sectional view of a tracking system illustrated in the fig.5 during a contact phase, in which the tip of the utensil is not rounded; the figure 4B is a schematic and partial view, in section, of a tracking system according to another variant embodiment, during a contact phase, in which the tip of the utensil is rounded. figure 5 is a schematic and partial perspective view of a system for tracking the tip of a utensil according to an alternative embodiment, during a non-contact phase. EXPOSÉ DÉTAILLÉ DE MODES DE RÉALISATION PARTICULIERS

[0023] In the figures and in the remainder of the description, the same references represent identical or similar elements. In addition, the different elements are not shown to scale so as to enhance the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and may be combined with each other. Unless otherwise indicated, the terms "substantially", "approximately", "of the order of" mean to within 10%, and preferably to within 5%. Furthermore, the terms "between ... and ..." and equivalents mean that the limits are included, unless otherwise indicated.

[0024] The invention relates to a system and method for tracking a first end of a utensil, for example a tip, provided with a magnetic object intended to be manipulated by a user. It is suitable for: determining a state vector XA representative of the position PA and the orientation of the magnetic object in an OXYZ frame of reference; determining the position PP of the tip of the utensil from the state vector XA and a recorded value of the distance D between the magnetic object and the tip; determining, when contact of the tip on a reference surface is detected, the effective value of the distance between the magnetic object and the tip (called the first distance and noted D 1 ), and updating the recorded value of the distance D 1 from the determined effective value.

[0025] To this end, the tracking system generally includes: a utensil provided with a magnetic object; a location device, comprising a memory containing a recorded value of the distance D 1 between the magnetic object and the tip, and adapted to determine the state vector XA representative of the position PA and the orientation of the magnetic object, and to determine a position PP of the tip of the utensil taking into account the recorded value of the distance D 1; a touch sensor having a reference surface, adapted to detect the contact of the tip on the reference surface; a graphical interface, when it is desired to display the position of the tip or another position resulting from the position of the tip, in particular during a non-contact phase.

[0026] The location device is equipped with a network of magnetometers to measure the ambient magnetic field, the contribution of which is generated by the magnetic object. Furthermore, the tactile sensor is said to be 'tactile' insofar as it is adapted to detect the contact of the tip of the utensil on the reference surface. It is then adapted to provide an electrical response signal representative of the possible contact of the tip of the utensil on the reference surface. The tactile sensor can be capacitive or resistive, for example piezoresistive. More broadly, the tactile sensor has a parameter that varies depending on the contact of the tip on the reference surface, and possibly the pressure force exerted. This parameter can be a capacitance, an electrical resistance, an electrical voltage, etc.The touch sensor can be 'matrix' when it has a matrix of pixels distinct from each other formed by conductive tracks arranged in rows and columns. Each pixel is then adapted to provide a response signal.

[0027] The tracking system can find an application in the digitization of a drawing or writing, which then consists of the recording and recording of a trace made by the tip of the utensil on the reference surface. By recording the trace made by the tip of the utensil, we mean the determination and recording of the successive positions of the tip of the utensil when it is in contact with the reference surface. It can also find an application, among other things, in human-machine interfaces, for example to interact with 2D or 3D digital objects displayed by the graphical interface.

[0028] As explained below, the method and the system for tracking the tip of the utensil are adapted, during a phase of contact of the tip on the reference surface, to determine the effective value D 1(k+1) of the distance D 1 between the magnetic object and the tip of the utensil, along the longitudinal axis thereof, and to update the recorded value of this distance D 1 in a calculation unit by replacing the previously recorded value D 1(k) by the determined effective value D 1(k+1). These steps of determining the effective value D 1(k+1) of the distance D 1 and updating the recorded value form a recalibration of the distance D 1 between the tip and the magnetic object.

[0029] In the context of the invention, the position of the magnetic object with respect to the tip (and therefore the distance D 1 ) is modified during the manipulation of the utensil, by translation of the magnetic object along the longitudinal axis Δ of the utensil, while the utensil and therefore the tip in question is not in contact with the reference surface. It is when the touch sensor detects contact of the tip on the reference surface that the recalibration of the distance D 1 is carried out.

[0030] THE figures 1 And 2are schematic and partial views, respectively in section and in exploded view, of a tracking system 1 of a tip 4 of a utensil 3 according to one embodiment. In this example, the utensil 3 is a stylus of which a first end 4 (here a tip) is intended to come into contact with the reference surface 2. Furthermore, the tactile sensor 20 is a resistive pressure matrix sensor, which is then adapted, in addition to detecting the contact of the tip 4 on the reference surface 2, to measure the pressing force (also called pressure force). However, as mentioned previously, the tactile sensor 20 may not be a matrix sensor.

[0031] The reference surface 2 may be the surface of a protective layer of the touch sensor 20 described in detail later. It may also be the surface of an added element arranged on a support surface of the touch sensor 20, this element being adapted to transmit the pressure force exerted by the tip 4 of the stylus 3 to the touch sensor 20. Such an element may be, for example, one or more sheets of paper.

[0032] Here and for the remainder of the description, we define a direct three-dimensional orthogonal reference frame OXYZ, where the X and Y axes form a plane parallel to the reference surface 2, and where the Z axis is oriented towards the utensil 3. In this example, the origin O is located at the edge of a tracking zone Zs of the magnetic object 6, and in a plane passing through the network of magnetometers, but it can be located elsewhere in the tracking zone Zs, for example at the edge and in the plane of the reference surface 2.

[0033] The utensil 3 is an object intended to be handled by a user, for example by hand. It comprises a rigid structure 5 (body) made of a preferably non-magnetic material, for example plastic, which has a tip 4 intended to come into contact with the reference surface 2 of the touch sensor 20. It may be a pencil in the broad sense, that is to say a pen, stylus, felt-tip pen, brush or any other writing or drawing device. The tip 4 is, generally speaking, one end of the utensil 3 along the longitudinal axis Δ, and may be pointed or rounded, rigid or deformable. By deformable, we mean here slightly deformable so that, when the point 4 comes into contact with the reference surface 2, the deformation of the tip 4 does not substantially modify the effective value of the distance D between the magnet 6 and the point 4 along the longitudinal axis Δ.In other words, within the framework of the invention, the modification of the value of the distance D is essentially due to the translation of the magnet 6 along the longitudinal axis Δ during the manipulation of the utensil 3 during a non-contact phase.

[0034] The rigid structure 5 of the stylus 3 extends along a longitudinal axis Δ. In other words, the rigid structure 5 comprises a front part (the tip 4) intended to come into contact with the reference surface 2, and a rear part, these front and rear parts being opposite each other along this longitudinal axis Δ.

[0035] The magnetic object 6, here a permanent magnet, is secured to the rigid structure 5 of the stylus 3. The magnetic object 6 comprises a material having a magnetization, for example remanent, for which a magnetic moment m is defined. It may be a cylindrical permanent magnet, for example annular, as illustrated in document WO2014 / 053526, or even an electromagnet. It may also be a transponder adapted to re-emit a magnetic field emitted by a network of magnetic generators. However, in this example, the magnetic object 6 is a permanent magnet.

[0036] The magnetic material is preferably ferrimagnetic or ferromagnetic. It has a non-zero spontaneous magnetic moment even in the absence of an external magnetic field. It may have a coercive magnetic field greater than 100 Am -1< or 500 Am -1< and the intensity of the magnetic moment is preferably greater than 0.01 Am 2< or even 0.1 Am 2< , for example equal to approximately 0.2 Am 2<. It is considered that the magnetic object 6 can be approximated by a magnetic dipole, but other models can be used. The magnetic axis of the magnetic object 6 is defined as being the axis collinear with the magnetic moment m of the magnetic object 6. The magnetic moment m is here collinear with the longitudinal axis Δ of the stylus 3.

[0037] The magnet 6 is integral with the rigid structure 5 in the sense that it cannot move alone in translation along the longitudinal axis Δ, but it is possible that the magnet 6 is nevertheless moved in translation by the user when handling the utensil 3 while the latter (and therefore the tip 4) is not in contact with the reference surface 2. In this case, the effective value of the distance D 1 between the magnet 6 and the tip 4 along the longitudinal axis Δ is then modified with respect to an initial value D 1 (k = 0) pre-recorded in the calculation unit 11 of the tracking system 1. This distance D 1 is defined as being the length along the longitudinal axis Δ between the barycentric center of the magnet 6 and the tip 4 of the stylus 3. This is why the tracking method provides a phase of recalibration of the distance D 1 .

[0038] The magnet 6 is initially placed at a non-zero distance D 1 (k=0) from the tip 4 of the stylus 3. This initial value D 1 (k=0) is known and recorded in the memory 13 of the calculation unit 11 of the location device 10, with a digital model representative of the utensil 3 used. Also, knowledge of the position PA and the orientation of the magnet 6 in the OXYZ frame of reference, and knowledge of the recorded value D 1 (k=0) of the distance D 1 , make it possible to deduce the position P p of the tip 4 of the stylus 3.

[0039] However, since the value of this distance D 1 can vary over time during the manipulation of the utensil 3 by the user during the non-contact phases and therefore have an effective value different from the initial value D 1(k=0), the tracking method comprises a determination of the effective value D 1(k+1), and an updating of the distance D 1 in the calculation unit 11 by replacing the previously recorded value D 1(k) with the determined effective value D 1(k+1). The subsequent determination of the position PP of the tip 4 is carried out from the recorded value D 1(k+1) then updated. The indicator k is incremented by one unit at each new phase of contact of the tip 4 on the reference surface 2.

[0040] The tracking system 1 also comprises a location device 10, adapted to locate the magnet 6, that is to say to determine a state vector XA representative of the position PA and the orientation of the magnet 6 in the OXYZ frame of reference, then to determine the position PP of the tip 4 in this same frame from the state vector XA and the recorded value of the distance D magnet 6 - tip 4. It is also adapted to determine, during a contact phase detected by the touch sensor 20, the effective value D 1(k+1) of the distance D 1 between the magnet 6 and the tip 4 and to update the recorded value from the determined effective value D 1(k+1).

[0041] The PP position in the OXYZ reference frame or a PI or PV position can then be displayed on the graphical interface 7, in particular during non-contact phases. The PI position is that of the point of intersection between the reference surface 2 and a straight line collinear with the magnetic moment of the magnet 6 and therefore passing through the tip 4. The PV position is the projection onto the reference surface 2, along the Z axis, of the PP position. The PP position can be displayed for example in the case of a three-dimensional representation of a virtual space containing digital objects to be manipulated (human-machine interface). The PV position and / or the PI position can be displayed for example in the case of a cursor being displayed on a two-dimensional representation of the reference surface 2 as part of the trace recording.

[0042] By locating the magnet 6, we mean determining the position PA and the orientation of the magnet 6 in the tracking zone Zs in the form of a state vector XA . The position PA of the magnet 6 corresponds here to the coordinates of the geometric center of the magnet 6, i.e. to the unweighted barycenter of all the points of the magnet 6. Furthermore, the magnetic moment m of magnet 6 is a vector whose components are (mx , my , mz ) in the real frame OXYZ. Its norm, also called intensity or amplitude, is noted || m || or m. It is collinear with the longitudinal axis Δ and therefore passes through the tip 4 of the stylus 3.

[0043] Magnet 6 is intended to move in the tracking zone Zs. This is a space in which the signal-to-noise ratio SNR (for Signal to Noise Ratio, in English) of at least one of the magnetometers of the location device 10 is greater than or equal to a predefined threshold value. For example, the tracking zone Zs may be a space in which the signal, i.e. the standard or at least one component of the magnetic field generated by the magnet 6 and measured by the corresponding magnetometer, is greater than or equal to, for example, 20 times the noise. The noise associated with each magnetometer may be equal to approximately 0.2µT. In this case, the tracking zone Zs corresponds to an area of space in which the magnetic field generated by the magnet 6 and measured by at least one of the magnetometers M i is greater than or equal to approximately 6µT, which corresponds to a distance d max equal to approximately 20cm along the director axis passing through the magnetometer M i considered.More simply, the tracking zone Zs can be defined as a space in which each point is at a distance less than or equal to a maximum distance d max along the director axis passing through the nearest magnetometer M i, this being for example equal to 20cm, or even 10cm, or even 5cm.

[0044] The location device 10 is capable of measuring the ambient magnetic field, one of the contributions of which is the magnetic field generated by the magnet 6, at different measurement times, during a tracking duration T, in the OXYZ frame of reference, and then of estimating the position PA and the orientation of the magnet 6 on the basis of the measurements of the magnetometers M i , and finally of determining the position PP of the tip 4 of the magnet 6 in the OXYZ frame of reference.

[0045] For this, it comprises a network of magnetometers M i secured here without degree of freedom to a rear face of the touch sensor 20. The number of magnetometers M i may be, for example greater than or equal to 2, preferably greater than or equal to 16, for example equal to approximately 25, in particular when it is a question of triaxial magnetometers. The network of magnetometers M i however comprises at least three measuring axes distant from each other and not parallel two by two. The magnetometers M i may be aligned rows and columns, or may be mutually positioned in a substantially random manner. The positions of the magnetometers M i are known. For example, they may be between 1 cm and 10 cm, for example 5 cm.

[0046] The magnetometers M i each have at least one measurement axis, for example three axes, denoted xi , yi , zi . Each magnetometer therefore measures the amplitude and direction of the ambient magnetic field B i , a contribution of which is generated by the magnet 6. More precisely, each magnetometer M i measures the norm of the orthogonal projection of the ambient magnetic field B i along the axes xi , yi , zi of the magnetometer. A calibration parameter of the magnetometers M i can be the noise associated with the magnetometers, here of the order of 0.2µT. By ambient magnetic field B, we mean the magnetic field not disturbed by any magnetic element, formed in particular by a terrestrial contribution B terr< of the order of 50µT, to which is added the magnetic field B a< generated by the magnet 6. Other magnetic contributions can be added, such as a contribution associated with the noise of the sensors and a contribution linked to an offset error ( offset, in English), which are neglected here.

[0047] The location device 10 further comprises a calculation unit 11 capable of: determining the state vector XA (position PA and orientation of the magnet 6 in the OXYZ frame), from the measurements of the magnetometers M i ; determining the position PP of the tip 4 of the stylus 3 in the OXYZ frame, from the state vector XA and the recorded distance D between the magnet 6 and the tip 4 ; determining, when the contact of the tip 4 on the reference surface 2 has been detected by the touch sensor 20, the effective value D 1(k+1) of the distance D 1 between the magnet 6 and the tip 4, and updating the recorded value of this distance D 1 .

[0048] For this, each magnetometer M i is electrically connected to the calculation unit 11 by an information transmission bus (not shown). The calculation unit 11 comprises a programmable processor 12 capable of executing instructions recorded on an information recording medium. It further comprises a memory 13 containing the instructions necessary for implementing the location of the magnet 6, as well as the digital model of the utensil 3 used, making it possible to provide the position P p of the tip 4 of the stylus 3 in the OXYZ frame of reference from the state vector XA . It therefore contains an initial value D 1(k=0) of the distance D1 between the magnet 6 and the tip 4, this value then being updated at each new contact phase and recorded in the calculation unit 11 (it is then noted D 1(1+1) ).Preferably, the initial value D 1(k=0) is small compared to the length of the stylus 3, so that the magnet 6 is initially positioned close to the tip 4. It can thus be substantially equal to half the length of the magnet 6. The memory 13 is also adapted to store the information calculated at each measurement instant. The calculation unit 11 is connected to the graphical interface 7 to display the position PP or a projected position PI or PV.

[0049] The computing unit 11 implements a mathematical model associating the position of the magnet 6 in the OXYZ frame, as well as the orientation and intensity of its magnetic moment m, to the measurements of the magnetometers M i . This mathematical model is constructed from the equations of electromagnetism, in particular magnetostatics, and is parameterized in particular by the positions and orientations of the magnetometers in the OXYZ frame. Here, this model is non-linear. The computing unit 11 implements an algorithm for estimating its solution such as, for example, Bayesian filtering (e.g. extended Kalman filter) or optimization, or even any other algorithm of the same type.

[0050] Preferably, in order to be able to approximate the magnet 6 to a magnetic dipole, the distance between the magnet 6 and each magnetometer M i is greater than 2, or even 3 times the largest dimension of the magnet 6. This dimension may be less than 20 cm, or even less than 10 cm, or even 5 cm. The magnet 6 may be modeled by a dipole model, among other things, depending in particular on the distance between the magnet 6 and each magnetometer M i of the network.

[0051] The tracking system 1 comprises a touch sensor 20, here a pressure matrix sensor. It is adapted to detect the contact of the tip 4 of the stylus 3 on the reference surface 2 from here the measurement of the intensity of the pressure force exerted on this reference surface 2, and to transmit this information to the location device 10. Thus, from this information, the tracking system 1 determines whether the manipulation of the stylus 3 is in a phase of contact of the tip 4 with the reference surface 2 or in a phase of non-contact.

[0052] The touch sensor 20 here comprises a pressure matrix, formed of a plurality of pixels Px i sensitive to the pressure exerted on its surface. The pressure matrix is in this example of the resistive type. Such a sensor is also called a resistive force sensor ( Force-Sensing Resistor, in English).

[0053] It is formed of a film 23 made of a piezoresistive material, that is to say a material whose local electrical resistance varies according to the mechanical stress exerted, for example a conductive polymer. The sensitive material of the film can be continuous in the XY plane or can be pixelated. Conductive tracks 21, 22 are formed in rows on one face of the film and in columns on the opposite face. The pixels Px i are formed by the intersection, in top view, between the rows and columns of the conductive tracks. The pixels Px i can be placed next to each other, or even be spaced apart from each other (as illustrated in the fig.2 ).

[0054] The touch sensor 20 here comprises a number N of pixels Px i , for example equal to approximately 2500. The conductive tracks 21, 22 may have a width of a few millimeters, for example 2.5 mm, so that a pixel here has a surface area of 2.5 × 2.5 mm 2 < . The pixels Px i are distinct and are spaced apart in the XY plane from each other, for example by a distance of approximately 1 mm. The diameter of the tip 4 of the stylus 3 in contact with the reference surface 2 may here be of the order of a millimeter, for example between approximately 1 mm and 3 mm.

[0055] The touch sensor 20 comprises a processing unit 24, comprising, for example, a microcontroller 25 ensuring the control and reading of the different pixels of the pressure matrix, and a computer 26 adapted to detect the contact of the tip 4 on the basis of the electrical response signals emitted by the pixels. The microcontroller is thus adapted to transmit an electrical control signal to each of the pixels, and to receive an electrical response signal, the latter being representative of a possible contact of the tip 4 of the stylus 3 on the reference surface 2. The processing unit comprises the coordinates of the N pixels Px i in the OXYZ reference frame.

[0056] The computer 26, from at least one response electrical signal, detects whether or not there is contact of the tip 4 on the reference surface 2. For this, the intensity of the electrical signal emitted by each pixel interrogated or, for example, a combination of the intensity of the electrical signal from each of the pixels interrogated, has a value which, when it is greater than a predetermined threshold, corresponds to the contact of the tip 4 on the reference surface 2. The intensity of the pressure force exerted by the tip 4 is dependent on the intensity of the electrical signals emitted by one or more pixels.

[0057] In the case of a trace survey, the calculation unit 11 of the location device 10 then records the successive positions P p (tn ) of the tip 4 in contact with the reference surface 2, determined by the location device 10, when the contact has been detected by the touch sensor 20, which thus form or participate in forming a part of the trace of the utensil 3 on the reference surface 2. All of the “surveyed” parts, taking into account the discontinuities (non-contact phases), form the trace of the stylus on the reference surface. The position PP then substantially coincides with the projected positions PI and PV . The intensity of the pressure force exerted (when it is greater than the threshold value for detecting the contact) makes it possible to provide an additional characteristic of the trace of the utensil 3, and can be used to vary, for example, the width of the trace of the utensil 3.

[0058] The processing unit 24 can thus comprise a microcontroller 25 for controlling the electrical power supply of the pixels, associated with analog-digital converters CAN, and its computer 26 comprises at least one processor and at least one memory containing the instructions necessary for implementing the detection of the contact of the tip 4 and the measurement of the pressure force, and for storing the information calculated at each measurement instant, and here converters. Of course, the processor and the memory of the computer 26 may or may not be common with those of the microcontroller 25, and / or with those of the location device 10.

[0059] As detailed below, the stylus 3 is intended to be manipulated by the user in a tracking zone Zs. At each measurement instant, the position PP of the tip 4 of the stylus 3 can be represented on the graphical interface 7. This manipulation involves the alternation of non-contact phases and contact phases of the tip 4 on the reference surface 2: during a first non-contact phase, the tip 4 of the stylus 3 is not in contact with the reference surface 2. The position PP of the tip 4 is then determined by the location device 10 from the state vector XA (position PA and orientation) of the magnet 6, and from the initial value D 1 (k = 0) of the distance D 1 between the magnet 6 and the tip 4 pre-recorded in the calculation unit 11 of the location device 10. The position PP of the tip 4 in the OXYZ frame of reference or a projected position PI or PV on the reference surface 2 can be represented in real time on the graphical interface 7; during a first contact phase, the user brings the tip 4 of the stylus 3 into contact with the reference surface 2. The touch sensor 20 then detects the contact, and the indicator k is incremented by one unit: k + 1. The actual value D 1(k+1) of the distance D 1 is then determined and the recorded value of the distance D 1 is then updated.The position PP of the tip 4 is determined from the state vector XA at the current time and from the updated value D k+1 of the distance D 1 between the magnet 6 and the tip 4; during a non-contact phase which follows the first contact phase, the position PP of the tip 4 is then determined from the state vector XA at the current time and from the already updated value D 1(k+1) of the distance D 1 between the magnet 6 and the tip 4.

[0060] The increment k increases by one unit at each new contact phase. A contact phase is defined as the period comprising one or more consecutive measurement instants during which the tip 4 of the stylus 3 is in continuous contact with the reference surface 2. The tracking system 1 can thus detect the initial instant of the first contact of the tip 4 on the reference surface 2 defining the start of a contact phase, as well as the final instant corresponding to the break in contact and defining the end of the contact phase.

[0061] Thus, the tracking system 1, by the combination of the location device 10 and the touch sensor 20, makes it possible to take advantage of the contact of the tip 4 on the reference surface 2 to determine the effective value D 1(k+1) of the distance D 1 between the magnet 6 and the tip 4, and to update the recorded value of this distance D 1 by replacing the previously recorded value (D 1(k) ) by the determined effective value (D 1(k+1) ). Thus, during each non-contact phase following a contact phase, the position PP of the tip 4 is determined by taking into account the recorded effective value D 1(k+1) of the distance D 1 between the magnet 6 and the tip 4 which was determined during the previous contact phase. This limits errors in determining the position of the tip 4, linked to an involuntary movement of the magnet 6 by the user.The accuracy of the location of the PP position of the tip 4 of the magnet 6 is then improved.

[0062] Furthermore, the contact of the tip 4 is detected precisely by the touch sensor 20, and the position of the tip 4 is determined with high resolution by the location device 10. The position of the tip 4 is thus not impacted by the potentially low resolution of conventional touch sensors, in particular matrix sensors whose resolution depends on the dimensions and arrangement of the conductive tracks. In addition, the determination of a projected position PI or PV on the writing surface, if applicable, is not disturbed by possible parasitic contacts, such as the contact of the finger or that of the palm of the user's hand on the reference surface 2.

[0063] Furthermore, the tracking system 1 may have a particularly reduced latency related to the control / reading of the pixel matrix, insofar as only the pixel or pixels closest to the projected position PI or PV can be activated by the microcontroller, and not all the pixels of the matrix. The control / reading frequency can then be particularly high. As a result, the power consumption of the tracking system 1 is reduced. In addition, it is advantageous for the touch sensor 20 to remain at least partly inactive during the tracking of the magnetic object 6 by the location device 10, in particular when the position PP of the tip 4 along the Z axis is greater than a threshold value PP,th.

[0064] Furthermore, it is possible to use a touch sensor 20 whose microcontroller and electrical connections are simple and conventional. This avoids having to use a microcontroller and special connections linked to the need to activate the rows and / or columns either individually and sequentially, or collectively.

[0065] There figure 3 is a flowchart illustrating a method for tracking the utensil 3 according to one embodiment. In this example, the tracking system 1 is identical to that described with reference to fig.1 And 2. In this example, the touch sensor 20 is activated regardless of the height of the tip 4 relative to the reference surface 2, and all of the pixels are interrogated. As described later, alternatively, the touch sensor 20 may only be activated when the position PP of the tip 4 along the Z axis is less than a threshold value, and only the pixel(s) closest to a projected position PI or PV on the reference surface 2 may be activated to detect the contact.

[0066] In a first step 11, a digital model of the stylus 3 is recorded in the memory of the calculation unit 11 of the location device 10. This digital model includes the initial value D 1 (k=0) of the distance D 1 between the magnet 6 and the tip 4. This recorded value is intended to be updated during each phase of contact of the tip 4 on the reference surface 2. It makes it possible to determine, before the first phase of contact, the position PP of the tip 4 of the stylus 3 in the OXYZ frame of reference from the state vector XA of the magnet 6. This step also includes the recording of the coordinates of the reference surface 2 in the OXYZ frame of reference, and the recording of the coordinates of the pixels of the touch sensor 20.

[0067] In a step 21, the user manipulates the stylus 3 in the tracking zone Zs, i.e. he modifies its position and possibly its orientation in the OXYZ reference frame. When manipulating the stylus 3, there is an alternation of contact and non-contact phases of the tip 4 on the reference surface 2. During a non-contact phase, the magnet 6 is moved (for example involuntarily by the user) by translation along the longitudinal axis Δ so that the actual value of the distance D 1 is different from the recorded value D 1(k=0).

[0068] The following steps 22 to 43 are performed iteratively at a measurement time tn, the time being discretized at a determined sampling frequency, for example 140Hz. Each iteration of rank n is associated with a measurement time tn, also called the current time. Furthermore, during each new contact phase, the indicator k is incremented by one unit.

[0069] During a step 22, the magnetometers measure the ambient magnetic field at the current time tn, and in particular the contribution of the ambient magnetic field generated by the magnet 6 secured to the stylus 3.

[0070] In a step 23, the calculation unit 11 receives the measurements of the ambient magnetic field, deduces therefrom the contribution of the magnetic field generated by the magnet 6, and determines the state vector XA (tn) associated with the magnet 6 at the current time tn in the OXYZ frame. The state vector XA (tn) comprises the position and orientation of the magnet 6 in the OXYZ frame. This estimation of the state vector XA can be carried out using an algorithm for estimating the position and orientation of the magnet 6 of the Bayesian type, for example an extended Kalman filter, or using an optimization method (gradient descent, etc.), or using any other algorithm of the same type. An example of estimation of a state vector associated with a magnet 6 is described in particular in application WO2018 / 219891.

[0071] Furthermore, a direction is given for the continuation of the monitoring process. +m Or -mat point 4. In other words, we determine which orientation +m Or -m is directed towards tip 4. For this, for example, the user can indicate that he wishes to follow and use tip 4 of stylus 3, stylus 3 can then be held by the user so that tip 4 is directed towards reference surface 2. The calculation unit 11 then determines whether the magnetic moment m is oriented towards the tip 4 or towards the opposite end 4', and attributes this orientation ( +m Or - m ) at point 4. Thus, knowing the orientation +m Or -m of the magnetic moment m with respect to point 4, we know whether the end of the stylus 3 in contact with the reference surface is the tip 4 or the opposite end 4'. In the rest of the description, we consider that the orientation +m of the magnetic moment is directed towards tip 4.

[0072] In a step 24, the touch sensor 20 determines whether or not there is contact of the tip 4 on the reference surface 2. In this example, the entire pixel matrix is then activated. For this, the processing unit determines the value of a parameter κ(tn ) representative of the contact of the tip 4 on the reference surface 2. This parameter is here the intensity of the electrical response signal of the pixels. It can be the electrical resistance of the pixels, or any other equivalent parameter (average intensity or average resistance of a set of pixels, etc.). The processing unit compares the value of the parameter κ(tn ) to a predefined threshold value κ th and detects the contact of the tip 4 on the reference surface 2 as being made when the value of the parameter κ(tn ) is greater than or equal to the threshold value κ th . The threshold value κ th makes it possible to filter out fluctuations linked to measurement noise.When contact is detected, the tracking process proceeds to step 31, and when not, it continues with step 41.

[0073] In step 31, when the contact is detected by the touch sensor 20, the location device 10 determines the effective value D 1(k+1) of the distance D 1 between the magnet 6 and the tip 4. For this, the touch sensor 20 has therefore detected a contact of the tip 4 on the reference surface 2. A contact phase therefore takes place and this information is transmitted to the location device 10. The calculation unit 11 then increments the indicator k by one unit: k+1. This indicator here keeps the same value as long as we remain in the same contact phase.

[0074] To determine the effective value D 1(k+1) of the distance D 1 , the calculation unit 11 determines a value D 1I(k+1) of the so-called projected distance D 1I separating the position PA of the magnet 6 and the position PI of the point I, the point I being defined as being the point of intersection between the reference surface 2 and the straight line parallel to the magnetic moment m and therefore passing through the tip 4. The coordinates of the reference surface 2 were recorded in the memory of the calculation unit 11 during step 11. Furthermore, it is considered that the vector OP I going from point O to point PI is the vector sum of the vector OP A going from point O to point PA (center of magnet 6) and the vector P A P I . From the determined value D 1I(k+1) of the projected distance D 1I , we determine the effective value D 1(k+1) of the distance D 1 , and we update the value recorded in the calculation unit.

[0075] Two situations can arise during a contact phase, which depend on the shape of the tip 4. A first situation is illustrated on the figure 4A and corresponds to the case where the tip 4 has a non-curved shape in the sense that there is coincidence, during contact, between the position PP of the end of the tip 4, the position PI of the point I, and the contact position P c. Note that the tip 4 may in fact have a certain curvature but which is sufficiently weak with respect to the thickness of the line represented so that we consider that there is coincidence between the three positions. A second situation is illustrated on the figure 4B and corresponds to the case where the tip 4 has a curved shape in the sense that there is a non-coincidence, during contact, between the position PP of the end of the tip 4, the position PI of the point I, and the position P c of contact.

[0076] In the case of the first situation ( fig.4A ), the effective value D 1(k+1) of the distance D 1 is equal to the value of the projected distance D 1 . We can write in a general way: ∥P A P I (tn )∥ = h A,z (tn ) × || m || / mz (tn ), to the absolute value. In other words, at the current time tn , the distance ∥ P A P I ∥ is equal to the height h A,z of the position PA of the magnet 6 with respect to the reference surface 2 along the Z axis, multiplied by the norm || m || of the magnetic moment m divided by the mz coordinate of the magnetic moment m along the Z axis. However, during a contact phase, the effective value D 1(k+1) (tn ) of the distance D 1 (just like the value D 1I(k+1) of the projected distance D 1I ) is equal to ∥ P A P I (tn )∥ : D k+1 (tn ) = ∥ P A P I (tn )∥.

[0077] Here, we consider that the value of mz is different from zero or is greater than a predefined threshold. If this is not the case, these steps may not be performed. We can thus not perform the determination and update of the distance D 1 when the magnetic moment has an inclination angle ϕ greater than 60° with respect to the Z axis. This limits the errors in the determination of the distance D 1 . In addition, as mentioned previously, we consider here that the orientation +m of the magnetic moment m is directed towards point 4.

[0078] Thus, the calculation unit 11 determines h A,z (tn ) from the position PA of the magnet 6 in the OXYZ frame determined previously during step 23 (determination of the state vector X a (tn )), and from the coordinates in the OXYZ frame of the reference surface 2. It also determines the norm || m || of the magnetic moment m . It therefore calculates the distance ∥ P A P I (tn )∥, that is to say the value D 1I(k+1) (tn ) of the distance D 1I , and thereby the effective value D 1(k+1) (tn ) of the distance D 1 between the magnet 6 and the tip 4.

[0079] In a step 32, the calculation unit 11 updates the recorded value of the distance D 1 between the magnet 6 and the tip 4 in the memory 13 by replacing the value D 1(k) with the actual value D 1(k+1). This recorded value can thus be an instantaneous value (e.g., the most recent) or be an average value of the different values of the distance D k+1 determined during the same contact phase. Other approaches are possible: it is possible, for example, to calculate the average by weighting the different determined values of D 1(k+1) as a function of the inclination of the stylus 3. Thus, a greater weight can be attributed to the determined values of D 1(k+1) when the stylus 3 forms an angle substantially orthogonal to the reference surface 2.

[0080] In step 33, the calculation unit 11 determines the position PP (tn ) of the tip 4 on the reference surface 2 at the current time, from the state vector XA (tn ) determined previously, and from the distance D 1 (k+1) updated from the distance D 1 . For this, the calculation unit 11 determines the vector PAPI (tn ) from the distance D 1 (k+1) updated and from the magnetic moment m (tn ): PAPI (tn ) = D 1 (k+1) × m (tn ) / ∥m∥. As mentioned previously, here we consider that the orientation +m of the magnetic moment m is directed towards tip 4. Then, the position P p (tn ) of tip 4 in contact with the reference surface 2 is determined from the vector PAPI (tn ) and the vector OP A (tn ): OP p (tn ) = OP I (tn ) = OP A (tn ) + PAPI (tn ).

[0081] In step 34, the calculation unit 11 records the position PP (tn). In the case of a trace recording application, the successive positions PP (tn) recorded by the calculation unit 11 during the contact phase form or participate in forming the trace made by the stylus 3 on the reference surface 2.

[0082] Alternatively, the position PP of the tip 4 in contact with the reference surface 2, as recorded by the calculation unit 11, may depend both on the position determined by the location device 10 (the position PI), and on a determined position PP,cp provided by the touch sensor 20. Thus, the position PP,cp (tn) may correspond to the position of the centroid (weighted barycenter) of the pressure forces exerted by the tip 4 on the reference surface 2, and measured by the interrogated pixels. The position of the tip 4 in contact with the reference surface 2 recorded by the calculation unit 11 may be the average, weighted or not as a function of the magnetic disturbance, of the position PP determined by the location device 10 and the position PP,cp determined by the touch sensor 20.

[0083] During a step 35, the calculation unit 11 controls the graphical interface 7 to display the position PP (tn) of the tip 4 (and where appropriate of the trace).

[0084] Furthermore, when the contact of the tip 4 on the reference surface 2 is not detected, steps 41 to 43 are performed.

[0085] In step 41, the locating device 10 determines the position PP (tn ) from the state vector XA (tn ) determined in step 23 and from the recorded value D 1(k) of the distance D 1 between the magnet 6 and the tip 4. The distance D 1(k) is the initial value D 1(k=0) when there has been no previous contact phase, or is the value D 1(k) determined and updated during the last contact phase. The position PP (tn ) can be determined as follows: OP P (tn ) = OP A (tn ) + D 1(k) × m (tn ) / ∥m∥.

[0086] During step 42, the location device 10 determines, if necessary, the projected position PV (tn ) which is the projection of the position PP (tn ) of the tip 4 on the reference surface 2 along the Z axis. This position PV (tn ) can correspond to the position of a cursor displayed on the graphical interface 7 in the case in particular of a trace reading. It is possible to choose as a variant to determine the position PI (tn ).

[0087] During step 43, the location device 10 controls the graphical interface 7 to display the position PP (tn) in the case in particular of an application of the human-machine interaction type and a three-dimensional representation of the stylus 3 or equivalent, or to display the position P v (tn) or the position PI (tn) in the case in particular of a trace recording application (or even the projected position PI).

[0088] Steps 22 to 43 are repeated at the defined sampling frequency, which may or may not be constant over time, and which may in particular depend on the speed of movement of the magnet 6 or of the tip 4, calculated by the location device 10 from the state vector XA (tn). In particular, the sampling frequency associated with the location of the magnet 6 (e.g. 140 Hz) may be identical to or different from the frequency of control / reading of the pixels of the touch sensor 20.

[0089] Thus, the tracking method makes it possible to regularly determine, at each contact phase, the effective value D 1(k+1) of the distance D 1 between the magnet 6 and the tip 4 of the stylus 3. Thus, the accuracy of the location of the position PP of the tip 4 of the magnet 6 is then improved. This limits the location errors in the case where the magnet 6 is moved from its initial position corresponding to the distance D 0 , and therefore the location errors of the tip 4 of the stylus 3.

[0090] There figure 4B is a schematic and partial view, in section, of a utensil 3 of a tracking system 1 according to an alternative embodiment, for which the tip 4 has a rounded shape. In this second situation, there is a non-coincidence, during contact, between the position PP of the end of the tip 4, the position PI of the point I, and the contact position P c.

[0091] It appears that when tip 4 has a curved shape (circular, oval or other), and the magnetic moment m is inclined relative to the Z axis by a non-zero inclination angle ϕ, the position PP does not coincide with the projected position PI, and that the contact of the tip 4 takes place at the effective contact position PC distinct from the position PI. Here, we consider that the spacing between the positions PI and PC is sufficiently large to be taken into consideration, for example when it is greater than the thickness of the line represented on a graphical interface. Otherwise, we return to the case of the first situation described on the fig.4A .

[0092] In this case where the inclination angle ϕ formed by the magnetic moment m with the Z axis is non-zero, tip 4 is formed by: an end zone located in the extension of the longitudinal axis Δ, and where the position PP is defined (located on the longitudinal axis Δ); and a contact zone, distinct from the end zone, at the level of which the contact with the reference surface 2 takes place, and where the effective contact position PC is defined.

[0093] The effective contact position P c can be determined from the position PI determined previously during step 33, and from information relating to the shape of the tip 4 pre-recorded during step 11. In this example where the tip 4 has a circular curved shape, this recorded geometric information relating to the shape of the tip 4 can include the value of a radius R of a sphere inscribed in the surface of the tip 4, and passing through the curved segment [PP; PC].

[0094] Thus, the effective value D 1(k+1) of the distance D 1 between the magnet 6 and the tip 4 can be calculated from the calculation of the value D 1I(k+1) of the projected distance D 1I and the recorded geometric information of the tip 4 (e.g. here: circular shape of radius R centered at PR) and the inclination angle ϕ formed by the magnetic moment m with respect to the Z axis. So, as an example, we can first calculate the distance ∥ P A P I (tn )∥ and deduce the position PI (tn ) ( OP I = OP A + P A P I ). Then, we calculate the position P c (tn ) from here the determined position PI (tn ), from the calculation of the vector e c whose norm is here equal to ec = Rxtan(ϕ) (the norm depends on the geometric information recorded on the shape of point 4) and the orientation corresponds to the projection of the magnetic moment min the XY plane. The inclination angle ϕ is calculated from the coordinates of the magnetic moment m . Then, the position P p (tn ) is calculated from, in particular, the position P c (tn ) and the geometric information of point 4 (and possibly the angle ϕ). Of course, this example is given for illustrative purposes, and there are different possibilities for determining the effective value D 1(k+1) of the distance D 1 from the distance ∥ P A P I (tn )∥ and recorded geometric information of the tip shape 4.

[0095] Alternatively and in the particular case of a point 4 of curved and circular shape, we can also calculate: ∥ P A P R (tn )∥ = (h A,z (tn )-R) × || m || / mz (tn ). We note h' A,z (tn ) the value h A,z (tn ) - R. Also, the calculation of ∥ P A P R ∥ indirectly corresponds to the calculation of ∥ P A P I ∥ and therefore of the value D 1I(k+1) of the distance D 1I . Also, at the current time tn , the distance ∥ P A P R ∥ of the vector going from the position PA of the magnet 6 to the position PR of the center of the inscribed circle is equal to the distance h' A,z of the position PA of the magnet 6 with respect to an XY plane passing through the position PR, multiplied by the norm || m || of the magnetic moment m divided by the mz coordinate of the magnetic moment m along the Z axis. The PR position is determined as follows: OP R = OP A + P A P R . After having determined the coordinates of the position PR in the OXYZ frame, we then directly obtain the position in the XY plane of the effective contact position P c: these are the coordinates in the XY plane of the position PR.

[0096] Other approaches are possible for determining the PC position, which depend for example on the geometric model of the shape of the tip 4 pre-recorded in the calculation unit 11 during step 11.

[0097] There figure 5 is a schematic and partial perspective view of a tracking system 1 similar to that described with reference to the fig.1 , in which the touch sensor 20 only activates the pixel(s) Px PI (tn ) closest to the position PI (tn ) of the intersection point I, and here when the position PP is less than or equal, along the Z axis, to a predefined threshold value PP,th.

[0098] Thus, the processing unit of the touch sensor 20 receives this data only when the position PP of the tip 4 along the Z axis is less than or equal to the threshold value PP,th. When this is not the case, the touch sensor 20 can be inactive, that is to say not fully or partially electrically powered, so as to limit the electrical consumption of the tracking system 1.

[0099] When the position PP is, along the Z axis, less than or equal to the threshold value PP,th , the touch sensor 20 is made active. It then receives the projected position PI or PV at the current time (here the position PI ), even though the contact is not detected.

[0100] The processing unit then determines the pixel(s) Px PI closest to the position PI. Only these pixels are then activated by the processing unit in order to detect the contact of the tip 4 on the reference surface 2.

[0101] The processing unit then measures only the nearest pixel Px ref and not all pixels in the matrix. The electrical control signal is therefore transmitted only to the nearest pixels Px PI while the other pixels are not electrically powered. Then, the electrical response signal is received by the processing unit. This significantly reduces the pixel control / reading latency, since only a few pixels are read for each measurement time, and not 2500 pixels. As a result, power consumption is also significantly reduced.

[0102] According to another embodiment, the end 4' of the stylus 3, opposite the tip 4 along the longitudinal axis Δ, can also be functionalized and thus be tracked by the tracking system 1. For example, the stylus 3 can have an eraser at the end 4', or a writing tip having characteristics different from those of the tip 4. Also, this end 4' can also be intended to come into contact with the reference surface 2.

[0103] As illustrated by the fig.1 , we note D 2 the distance between the magnet 6 and the end 4' along the longitudinal axis Δ. When, during the use of the stylus 3, the magnet 6 is moved with respect to its original position, and the distance D 2 then has an effective value different from its initial recorded value.

[0104] The tracking method is then adapted to ensure tracking, in addition to the tip 4, of the end 4' as well. The method is then similar to that described previously with reference to the fig.3 It is essentially distinguished by the following elements.

[0105] In step 11, the digital model of the stylus 3 recorded in the calculation unit 11 comprises, in addition to the initial value D 1(k=0) of the distance magnet 6 - tip 4, the initial value D 2(k'=0) of the distance magnet 6 - end 4'.

[0106] During step 21 of manipulation of the stylus 3, there is alternation between contact phases and non-contact phases of one of the ends 4 and 4' on the reference surface 2. A contact phase may concern the tip 4, and the following contact phase may concern the end 4' (these two successive contact phases being separated by a non-contact phase of the stylus 3 on the reference surface 2).

[0107] In step 23, an orientation of the magnetic moment is assigned for the rest of the tracking process. m at tip 4, and therefore the opposite orientation at the opposite end 4'. This step can be carried out as described previously. This is to determine which orientation +m Or -m of the magnetic moment m is directed towards point 4. Thus, knowing the orientation +m Or -m of the magnetic moment m , we know which end of the stylus 3 is in contact with the reference surface 2.

[0108] In step 24, the touch sensor 20 detects the contact of one end of the stylus 3 on the reference surface 2. Knowledge of the orientation +m Or -m of the magnetic moment mwith respect to tip 4 makes it possible to determine whether the end in contact is tip 4 or end 4'. Thus, the tracking system 1 "knows" which effective value of the distance D 1 or D 2 will be determined. For example, in the case where the orientation +m is directed towards the tip 4 and that it is here directed towards a direction opposite to the reference surface, the tracking system 1 determines that the contact concerns the end 4' and that it will determine the effective value of the distance D 2 . The processing unit then determines the value of a parameter κ'(tn ) representative of the contact of the end 4' on the reference surface 2. This parameter κ' is similar to the parameter κ except that it is associated with the end 4'.

[0109] During step 31, in this example where there is contact between the end 4' on the reference surface 2, the calculation unit 11 then determines the effective value of the distance D 2 magnet 6 - end 4', and then increments the indicator k' by one unit: k'+1. This indicator here keeps the same value as long as we remain in the same contact phase. Furthermore, the indicator k associated with the tip 4 remains unchanged.

[0110] To determine the effective value D 2(k'+1) of the distance D 2 , the calculation unit 11 determines the distance D 2I separating the position PA of the magnet 6 and the position PI of the point I, the point I always being defined as being the point of intersection between the reference surface 2 and the line parallel to the magnetic moment m and therefore passing through the end 4' (and through the point 4). In a similar way to what was described previously, the effective value of the distance D 2(k'+1) (tn ) is equal to ∥ P A P I (tn )∥ : D 2(k'+1) (tn ) = ∥ P A P I (tn )∥.

[0111] In step 32, the calculation unit 11 updates the recorded value of the distance D 2(k'+1) magnet 6 - end 4' in the memory 13.

[0112] During step 33, the calculation unit 11 determines the position PP' (tn ) of the end 4' on the reference surface 2 at the current time, from the state vector XA (tn ) determined previously, and from the updated distance D 2(k'+1).

[0113] The other steps are identical or similar to those described previously. Thus, at each contact phase, there is an update of the distance D 1 (magnet - tip 4) or of the distance D 2 (magnet - end 4'), and an increment of the corresponding indicator k or k'. This provides a stylus 3 whose two opposite ends 4, 4' can be tracked and whose contact of each of them on the reference surface 2 makes it possible to update its effective distance which separates it from the magnet 6. This limits the location errors, for each of these ends 4, 4', in the case where the magnet 6 is displaced with respect to its original position.

[0114] Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art.

Claims

1. Method for tracking a first end (4) of a utensil (3) handled by a user, a magnetic object (6) being securely fastened to the utensil (3) and remote from the first end (4) along a longitudinal axis △ of the utensil (3) by a first non-zero distance (D1), the magnetic object (6) having a magnetic moment m collinear to the longitudinal axis △, the method comprising the following steps: ∘ handling the utensil (3) by a user, the first end (4) being alternately in contact and not in contact with a reference surface (2), ∘ measuring a magnetic field emitted by the magnetic object (6) at different successive measurement instants, by means of a network of magnetometers (Mi) securely fastened to the reference surface (2); ∘ determining a state vector XA(tn) representative of the position PA(tn) and the magnetic moment m(tn) of the magnetic object (6), on the basis of the measurements of the magnetic field; ∘ determining the position Pp(tn) of the first end (4) of the utensil (3) on the basis of the state vector XA(tn) and of a previously recorded value (D1(k)) of the first distance (D1) between the first end (4) and the magnetic object (6); ∘ determining a parameter κ(tn) representative of a contact between the first end (4) and the reference surface (2) belonging to a touch sensor (20); characterized in that, the first distance (D1) is modified, during the handling of the utensil (3) while the first end (4) is not in contact with the reference surface (2), by translation of the magnetic object (6) along the longitudinal axis △ such that it has a value, referred to as the actual value, (D1(k+1)) that is different from the recorded value (D1(k)), the method comprises the following steps: ∘ when the contact is detected by the touch sensor (20): • determining a value (D1I(k+1)) of a first distance, referred to as the projected distance (D1I), which is equal to a distance between the position PA(tn) of the magnetic object (6) and a position PI(tn) of a point of intersection I between the reference surface (2) and a straight line collinear with the magnetic moment m(tn); then • determining the actual value (D1(K+1)) of the first distance (D1) on the basis of the determined value (D1I(K+1)) of the first projected distance (D1i); • updating the recorded value of the first distance (D1) by replacing the previous recorded value (D1(k)) with the determined actual value (D1(k+1)) of the first distance (D1).

2. Method according to claim 1, wherein the value (D1I(k+1)) of the first projected distance (D1I) is determined on the basis of a norm ∥PAPI(tn)∥ of a first vector PAPI(tn) going from the position PA of the magnetic object (6) to the position P1 of the point of intersection I, this norm ∥PAPI(tn)∥ of the first vector PAPI(tn) being determined on the basis of a norm ∥m∥ of the magnetic moment m, of the coordinate mz of the magnetic moment along a vertical axis orthogonal to the plane of the reference surface (2), and on the basis of a first distance hA,z along the vertical axis between the position PA of the magnetic object (6) and the reference surface (2).

3. Method according to claim 2, wherein, when the contact is detected, the position PI(tn) of the point of intersection I is determined on the basis of the position PA(tn) of the magnetic object (6) and on the basis of the first vector PAPI(tn), which is determined on the basis of the previously determined norm ∥PAPI(tn)∥, of the magnetic moment m and of the norm ∥m∥ of the magnetic moment.

4. The method according to claim 3, wherein, when contact is detected, the first end (4) having a curved shape, and the magnetic moment m having a non-zero angle of inclination Φ relative to the vertical axis, so that the first end (4) comprises an end region located in the extension of the longitudinal axis △, as well as a contact region, separate from the end region, at which the first end (4) is in contact with the reference surface (2), an actual contact position PC then being determined on the basis of the position PA and of the magnetic moment m of the magnetic object (6), and on the basis of at least one pre-recorded geometric parameter representative of the curved shape of the first end (4).

5. The method according to claim 4, wherein the pre-recorded geometric parameter is the center of a sphere of radius R, one surface of which is locally tangent to a surface of the first end (4) and passing through the position PP and the actual contact position PC, the actual contact position PC being determined on the basis of the magnetic moment m and of the norm ∥m∥ thereof and on the basis of a distance h'A,z along the vertical axis between the position PA of the magnetic object (6) and a plane parallel to the reference surface (2) and passing through the center of the sphere.

6. Method according to either claim 4 or claim 5, wherein the actual contact position Pc of the first end (4) is displayed on a graphical interface (7).

7. Method according to any of claims 1 to 6, wherein displayed on a graphical interface (7) is the position Pp(tn) of the first end (4), or a position of the first end (4) determined by projecting the position Pp(tn) on the reference surface (2) along an axis collinear with the magnetic moment m(tn) or along a vertical axis orthogonal to the reference surface (2).

8. Method according to any of claims 1 to 7, wherein the position Pp(tn) of the first end (4) is determined on the basis of the magnetic moment m, of the norm ∥m∥ thereof, and of the recorded value of the first projected distance (D1I(k+1)), or when no update of the first distance (D1) has yet occurred, of the pre-recorded value (D1(k=0)) of said first distance (D1).

9. Method according to any of claims 1 to 8, wherein the step of determining the first projected distance (D1I(k+1)) is not carried out when the magnetic moment m of the magnetic object (6) has an angle of inclination φ greater than 60° relative to a vertical axis orthogonal to the plane of the reference surface (2).

10. Method according to any of claims 1 to 9, wherein the touch sensor (20) is inactive as long as the position PP of the first end (4) is greater than a predefined threshold value PP,th along a vertical axis orthogonal to the plane of the reference surface (2), and is electrically activated when the position PP is less than or equal to the threshold value PP,th.

11. Method according to any of claims 1 to 10, the touch sensor (20) being a matrix sensor comprising a separate matrix of pixels securely fastened to the reference surface (2) and suitable for providing an electrical response signal representative of a contact of the first end (4) on the reference surface (2), the method comprising determining at least one pixel, referred to as the closest pixel, which has a distance relative to the position PI(tn) less than or equal to a predefined threshold value, only the closest pixel being activated in order to detect the contact.

12. Method according to any of claims 1 to 11, the utensil (3) comprising a second end (4') opposite the first end (4) along the longitudinal axis △, the magnetic object (6) being remote from the second end (4') along a longitudinal axis △ by a non-zero second distance (D2), the method comprising a step of determining the orientation +m or -m of the magnetic moment m directed toward the first end (4).

13. Method according to claim 12, additionally ensuring the tracking of the second end (4') of the utensil (3), the second distance (D2) between the second end (4') and the magnetic object (6) being able to be modified during handling, and comprising the following steps: ∘ during the step of handling of the utensil (3) by a user, the second end (4') is alternately in contact and not in contact with a reference surface (2) during the handling of the utensil (3) while the second end (4') is not in contact with the reference surface (2), the second distance (D2) is modified by translation of the magnetic object (6) along the longitudinal axis △ such that it has a second value, referred to as the actual value (D2(k'+1)), that is different from a second recorded value (D2(k')),; ∘ determining the position PP'(tn) of the second end (4') of the utensil (3) on the basis of the state vector XA(tn), of the previously recorded value (D2(k')) of the second distance (D2), and of the orientation of the magnetic moment m toward the second end (4'); ∘ determining a parameter k'(tn) representative of a contact between the second end (4') and the reference surface (2), and when contact is detected by the touch sensor (20): • determining a value (D2I(k'+1)) of a second distance, referred to as the projected distance (D2I), which is equal to a distance between the position PA(tn) of the magnetic object (6) and a position PI(tn) of a point of intersection I between the reference surface (2) and a straight line collinear with the magnetic moment m(tn), then; • determining the actual value (D2(k'+1)) of the second distance (D2) on the basis of the determined value (D2I(k'+1)) of the second projected distance (D2i); • updating the recorded value of the second distance (D2) by replacing the previous recorded value (D2(k')) with the determined actual value (D2(k'+1)) of the second distance (D2).

14. System (1) for tracking a first end (4) of a utensil (3), the system comprising: ∘ a utensil (3), • intended to be handled by a user to cause the first end (4) to be alternately in contact and not in contact with a reference surface (2), • a magnetic object (6) being securely fastened to the utensil (3) and remote from the first end (4) along a longitudinal axis △ of the utensil (3) by a first non-zero distance (D1) that may vary during the handling of the utensil (3) by a user while the first end (4) is not in contact with the reference surface (2) by translation of the magnetic object (6) along the longitudinal axis △ such that it has a value, referred to as the actual value (D1(k+1)), that is different from a recorded value (D1(k)), the magnetic object (6) having a magnetic moment m collinear with the longitudinal axis △; ∘ a locating device (10), comprising: • a network of magnetometers (Mi), securely fastened to the reference surface (2), and suitable for measuring a magnetic field emitted by the magnetic object (6); • an electronic computing unit (11), comprising a previously recorded value (D1(k)) of the first distance between the first end (4) and the magnetic object (6), and being suitable for - determining a state vector XA(tn) representative of the position PA(tn) and of the magnetic moment m(tn) of the magnetic object (6) on the basis of the measured magnetic field, - determining a position Pp(tn) of the first end (4) on the basis of the state vector XA XA(tn) and of a previously recorded value (D1(k)) of the first distance (D1); - when contact is detected between the first end (4) and the reference surface (2) by a touch sensor (20), determining a value (D1I(k+1)) of a first distance, referred to as the projected distance (D1I), which is equal to a distance between the position PA(tn) of the magnetic object (6) and a position PI(tn) of a point of intersection I between the reference surface (2) and a straight line collinear with the magnetic moment m(tn), then determining the actual value (D1(k+1)) of the first distance (D1) on the basis of the determined value (D1I(k+1)) of the first projected distance (D1i); and updating the recorded value of the first distance (D1) by replacing the previous recorded value (D1(k)) with the determined actual value (D1(k+1)) of the first distance (D1); ∘ the touch sensor (20), comprising the reference surface (2), suitable for detecting contact between the first end (4) and the reference surface (2), and for transmitting the contact information to the locating device (10).