Patent ID: 12253355

DETAILED DESCRIPTION OF THE INVENTION

With reference toFIG.1, a system50is described for tracking an object or a target1, i.e. to follow the evolution with time of the position of object1. System50essentially comprises three parts, i.e. a magnetic field source10, a data collection unit20and a data processing unit30.

Collection unit20comprises a support23on which magnetic field sensors22are fixed forming an array21. Therefore, magnetic field sensors22have predetermined positions with respect to support23, generally known apart from the normal mounting tolerances. Magnetic field sensors22are configured to detect a magnetic field at their own respective positions.

The magnetic field source can be schematized as a dipole10fixedly arranged on object1, or as a plurality of dipoles, in particular two dipoles fixedly arranged on object1, as described more in detail hereinafter. The position of object1is determined by identifying the position of dipole10by collection unit20and data processing unit30. In a generic point and, in particular, at points where sensors22can be arranged, dipole10generates a local magnetic field4whose magnetic induction vector B4can be calculated in a known way by knowing the magnetic moment of the dipole and the position of the point with respect to dipole10.

Normally, the magnetic field determined by sensors22include a contribution of earth's magnetic field or geomagnetic field5, which can be considered substantially homogeneous, i.e. uniform at least in all the space occupied by array21where the measurements are made. The module of magnetic induction vector B5of this geomagnetic field has a value that is normally set between 25 and 65 microtesla and that depends on the place on the Earth's surface where it is detected. In the presence of the magnetic field source or dipole10, the magnetic field also includes a contribution of local magnetic field4generated by dipole10. Local magnetic field4is not homogeneous, i.e., it changes with the position at which it is measured with respect to dipole10, in particular magnetic induction vector B4has a module depending on the inverse cube of the distance of the measurement point from dipole10, and its direction is given by field lines that are known when the shape and the position of dipole10are known.

According to the invention, magnet10is selected in such a way that, at the positions of magnetic field sensors22, the contribution B4of local magnetic field4generated by dipole10is comparable with the contribution B5of geomagnetic field5, i.e., in such a way that the modules of the two contributions have the same order of magnitude, or have contiguous orders of magnitude. This is obtained by using a dipole10that, in the above-described position range, generates a magnetic field whose intensity, i.e. the module of magnetic induction vector B4, is set between 5 and 500 microtesla.

InFIG.1an object to be tracked1is shown having a substantially spherical shape, rotatably arranged about three orthogonal axes x, y, z in a housing1′ that also has a spherical shape. However, this does not limit either the shape or the number of degrees of freedom, i.e. the possible movements of object1.

For example, magnetic field source i.e. dipole10can be provided by a Neodymium Nd magnet, in particular a 1 Tesla or about 800 kA/m Nd magnet.

The inventors have discovered that, by the magnetization level provided by such a magnet, an intensity of 5-500 microtesla at the positions of magnetic field sensors22of array21can be obtained if magnetic field source10is selected in such a way that the ratio between a linear dimension of array21of sensors22and a linear dimension of magnetic field source10is set between 40 and 60, preferably if this ratio is about 50, in particular, for a subset of at least three of sensors21, intended as triaxial vector sensors.

This provides a scale-up criterium for magnetic field source10and for data collection unit20of system50of the invention, apart from the obvious technological constraint due to the minimum distance that must be present between magnetic field sensors22to correctly detect their positions, and taking into account the dimensions of the integrated circuits, not shown, that include the sensors and/or must be added/arranged on support23in order to control the magnetic field sensors22themselves.

Magnetic field sensors22are preferably magnetoresistive sensors, even if different sensors can be used, for instance, fluxgate sensors. Preferably, magnetic field sensors22are vector sensors, i.e. each sensor unit is configured to provide three scalar values corresponding to the three components of magnetic induction vector B of the detected magnetic field.

A possible exemplary embodiment of support23is shown InFIG.2, which in the experiments has allowed particularly precise trackings. In this exemplary embodiment, support23comprises first and second support surfaces23aand23bparallel to each other, the sensors being distributed on both the support surfaces23aand23b. In particular, as shown inFIG.1, at least one magnetic field sensor22is fixed to second support surface23b, the other sensors arranged on first support surface23a. Even if support23shown in the drawings always comprises flat support surfaces23,23a,23b, other shapes are also possible, provided magnetic field sensors22are fixed to support23in relative predetermined positions, not lying on a same plane.

Moreover,FIG.2relates to a particular case in which array21comprises seven vector sensors22, even if the number of sensors can differ from the number which indicated here, which has the effects that will be described hereinafter. In this case, each magnetic field sensor22provides three scalar values that are the three components of magnetic induction vector B in a reference system integral to support23.

Still with reference toFIG.1, system50also comprises a sensor control unit for controlling magnetic field sensors22including a plurality of ports, not shown, for corresponding independent buses26′, for example I2C buses, each of which connects a magnetic field sensor22with a respective port of sensor control unit25, the set of the doors and of independent buses26′ constitutes a connection means26connecting magnetic field sensors22and sensor control unit25. This way, sensor control unit25can receive at the same time magnetic field data measured in the field by all magnetic field sensors22.

System50also comprises a data processing unit30configured to receive the raw magnetic field data from magnetic field sensor control unit25. Sensor control unit25and data processing unit30are distinct logical units, implemented according to various architectures.

As shown inFIGS.3and4, sensor control unit25can be implemented with a microcontroller25integral to support23or arranged on support23. For instance, the microcontroller can be a PIC18F47J53 type by Microchip, or other type having an appropriate number of digital input ports to provide the independent buses required by the number of sensors, and that has USB port or an equivalent port for connecting remote data processing unit30, as described hereinafter. Still as an example, the chip comprising sensors22can be a Isentek IST8308 type.

As diagrammatically shown inFIG.4, data processing unit30can also be integral to support23of data collection unit20, and can have a connection to an operator interface40, such as a display or a monitor40. Alternatively, as diagrammatically shown inFIG.3, data processing unit30can be implemented with a calculation device out of the data collection unit, for example a personal computer30or an equivalent device including a conventional operator interface means33. In the latter case, the connection between sensor control unit25and data processing unit30can be made by a conventional data transfer means, such as a USB cable37, which can work also serve as an electric connection means of magnetic sensor control unit25and, through the same, of magnetic field sensors22and other parts of data collection unit20, as referred to hereinafter.

The flowsheets ofFIGS.5-8describe a possible operation mode of magnetic field sensors22and of logical magnetic field sensor control unit25and of a control unit30of the data produced by magnetic field sensors22.

In particular, with reference toFIG.5, magnetic sensor control unit25, once it has received a data collection start command250from an operator, performs a step251of prearranging magnetic field sensors22according to configuration data249stored in a memory unit of the collection unit and, subsequently, starts an iterative continuous measurement step252comprising a plurality of identical measurement cycles252n, where n is a subscript counting the measurement cycles, each of said cycles including a sequence of steps or events as described hereinafter. The data collection start command250can be input by the operator in tracking mode or in calibration mode.

Once measurement step252has been started, sensors22perform a step221of detecting the magnetic induction vector at respective detection points, which is the resultant of the contribution of magnetic induction vector B4of local magnetic field4, generated by magnetic field source10, and of magnetic induction vector B5of geomagnetic field5(FIG.1). The detection step221outputs a set of raw magnetic field data222. These data are indicated this way because they are normally affected by offset and/or gain anisotropy issues, including such particular problems as offset and/or gain inhomogeneity of the sensors, offset and/or gain dependance on the temperature, inhomogeneous gain of the scalar components in the case of vector sensors. These effects can be more or less important, according to the type of sensor in use.

Sensor control unit25performs a step253of sampling raw magnetic field data222each time253b/YES an interval expires of a plurality of time intervals of predetermined duration stored in a timer253a. Upon each expiration of these time intervals, a step253cis performed of reading the lastly measured raw magnetic field data222.

Once sensor control unit25has obtained the new sample of raw magnetic field data222, sensor control unit25performs a step254of checking whether the start command has been input in “calibration” mode or in “tracking” mode.

In the first case, sensor control unit25performs a step255cof storing the current magnetic field datum222in a memory unit27′ (FIGS.3and4) of collection unit20as a datum of a plurality of calibration magnetic field data256c, i.e. magnetic field data intended for computing conversion parameters for normalizing raw magnetic field data222collected during subsequent uses of the system in tracking mode. It is assumed that, during the step of calibration, magnetic field data222are detected and collected while repeatedly modifying the orientation in the space of the array of sensors22, i.e. the orientation in the space of support23and, therefore, of collection unit20. Memory unit27′ can be, in particular, an EEPROM memory unit,

For each datum stored into memory unit27′, sensor control unit25performs a step257cof checking whether the stored data are sufficient or not, in comparison to a predetermined minimum amount of data. If not, a new measurement cycle252nis repeated while, in the opposite case, sensor control unit25performs a step258of discontinuing continuous detection221by magnetic field sensors22and measurement cycles252n.

Reverting to checking step254, if the start command was input in “tracking”

mode, sensor control unit25performs a step255tof storing current raw magnetic field datum222as tracking magnetic field datum256tinto a memory unit27″ (FIGS.3and4) of collection unit20that has the structure of a “first in first out” (FIFO) buffer.

For each datum stored into FIFO buffer27″, sensor control unit25performs a step257tof checking whether a tracking-stop command has occurred, In particular, input by the operator. If not, If not, a new measurement cycle252nis repeated while, in the opposite case, sensor control unit25performs a step258of discontinuing continuous detection221by magnetic field sensors22and measurement cycles252n.

FIG.6shows the operations carried out by data processing unit30when operating in “calibration” mode. Data processing unit30is configured to perform a process of computing conversion parameters310starting from a set of calibration raw magnetic field data256c, available in memory unit27′ of sensor control unit25, as described above (FIG.5), and recovered by data processing unit30through a step301of reading memory unit27′.

For example, if Vj,nindicates the values of the component oriented as the jthaxis of 3 coordinate axes in a nthof N stored data, the process comprises a step302of calculating a matrix Ti,j(k)of gain-related parameters and an offset vector Oj(k)that minimize the amount:
f=Σi,j,k,n=[B02−(Ti,j(k)Vj,n−Oj(k))2]2[1]
where each value of k=0 . . . (K−1) relates to a respective magnetic field sensor22, K is the total number of sensors in use of array21, i is a further subscript referring to the coordinate axes and B0is the module of the geomagnetic field that is detected while changing the orientation of array21of magnetic field sensors22, by which the calibration-intended magnetic field data256care collected.

In the ideal case that the axes of kthsensor are perfectly orthogonal, the matrix Ti,j(k)is a diagonal matrix in which Ti,j(k)=0 for i≠j, which has therefore only three non-zero coefficients that are the gain correction parameters of kthsensor along the three axes of it. In order to correct possible orthogonality defects of the three axes, for each sensor a step303is provided of calculating three further parameters corresponding to the elements of matrix Ti,j(k)for which i>j, so that that the matrix T becomes triangular. Finally, assuming that the reference system of array21is the one defined by the sensor of subscript k=0, for each sensor of subscript k>0 a rotation matrix Ri,j(k)is defined by three angles θ(k), φk), ψ(k). These further conversion parameters can be determined by minimizing the amount:
g=Σi,j,n(Bi,n(k)−Ri,jBj,n(0))2.  [2]

The process briefly described above makes it possible to determine nine conversion parameters for each magnetic field sensor22, i.e., three offset Oj(k), three gain values Ti,j(k), and three orthogonality defect compensation coefficients Ri,j, besides angles θ(k), φ(k), ψ(k), i.e. matrix R(k), for all the sensors with k>0.

In summary, by the analysis of calibration data a set of 12K−3 conversion parameters310is generated. A step follows of storing or saving304offset vector Oj(k), matrix Ti,j(k)of the gain-related parameters, and rotation matrix Ri,j(k)for each sensor k. The two matrix R(k), T(k)are indicated in the compact form of a conversion matrix C(k)=R(k)T(k). This way, conversion parameters are available for subsequently normalizing or converting tracking raw magnetic field data256t(FIG.5) into normalized data.

With reference toFIG.7, once magnetic field data processing unit30has received a tracking-mode start command from an operator, magnetic field data processing unit30starts a continuous and iterative processing step320through sensor control unit25, said step comprising a plurality of identical measurement cycles320n, with n is a subscript counting the processing cycles, each of said cycles including a sequence of steps or events as described hereinafter.

Data processing unit30firstly performs a step310of recovering conversion parameters calculated starting from calibration magnetic field data256cand then stored, as previously described (FIG.6), and a step324of reading tracking raw magnetic field data256t(FIG.5) from buffer27″. During the tracking process, sensor control unit25and data processing unit30can be working at the same time, in other words data processing unit30can receive tracking magnetic field data256tas these are provided by sensor control unit25of collection unit20, as described above.

Once data processing unit30has received conversion parameters310and tracking magnetic field data256t, data processing unit30performs a step325of normalizing tracking magnetic field data256t, i.e. a step of converting the latter from raw magnetic field data to normalized and homogeneous magnetic field data326, using conversion parameters310, in order to correct the errors due to offset and/or gain inhomogeneity between a sensor and the other, and to gain anisotropy, by which data256tare more or less affected, according to sensor type in use and to possible orthogonality defects of the three axes of the scalar sensors of each vector sensor22.

Once data processing unit30has obtained normalized magnetic field data326, i.e. the values of the components of magnetic induction vector B at the measurement point of each magnetic field sensor22, substantially free from the above described errors and all referred to the coordinate system associated with the sensor of index k=0, and once it has performed a step of recovering position data331of sensors22, data processing unit30can use normalized magnetic field data326to carry out a step of tracking or a tracking327.

To this purpose, data processing unit30is configured to perform a best-fit optimization process, in which a vector of position coordinates is calculated which minimizes the estimation error, i.e. the distance between the values of expected magnetic field Btheoat the positions of sensors22and the values of measured magnetic field Bmsrd, i.e. obtained from normalized magnetic field data326.

The values of magnetic field Btheoprovided at the positions of sensors22can be calculated by adding calculated expected values of local magnetic field4and of geomagnetic field5. The expected values of said local magnetic field4can be calculated starting from first guess values of the location and orientation coordinates of magnetic field source10, schematized as a magnetic dipole or as a combination of magnetic dipoles whose magnetic moment can be obtained by well-known relationships providing the magnetic induction vector generated by a dipole at a known position with respect to it. The expected values of said geomagnetic field at the positions of said magnetic field sensors22can be calculated starting from orientation coordinates of said geomagnetic field5.

For example, the estimation error can be calculated as the sum, hereafter indicated as “S”, extended to all magnetic field sensors22of array21and to the three cartesian directions, of the square of the differences between the measured components and the estimated components of magnetic induction vectorB:

S=∑i=13∑k=1K-1(Bi,msrd(k)-Bi,theo(k))2
where Bi,msrd(k)is the ithcomponent of the magnetic induction vector measured by the kthof K triaxial sensors, and Bi,theo(k)is the ithcomponent of the magnetic induction vector expected at the kthsensor, where the distance between the two vectors magnetic induction is the distance induced by the Euclidean norm, therefore the quadratic distance is minimized. When this error falls below a predetermined value, the coordinates of the current iteration are assumed as the coordinates of dipole10and of geomagnetic field5.

Tracking data330generated during the tracking step327can be used immediately to carry out a step332of updating the control display40of the data processing unit, for example a monitor40(FIGS.1,3,4), or can be used for a later display, once all the cycles320mof processing step320have been executed.

Once a tracking datum or a block of tracking data330has/have been calculated and displayed and/or recorded, data processing unit30performs a step333of checking whether a processing-stop condition has occurred or not, typically input by an operator. If not, a new processing cycle320mis repeated, while in the opposite case data processing unit30discontinues processing step320.

The flowsheet ofFIG.8also relates to the operation of data processing unit30to track object1. In an exemplary embodiment, before starting the continuous and iterative processing step320, a preliminary step321of filling a vector322of location and orientation coordinates of dipole10as well as of orientation coordinates of geomagnetic field5with first guess values for the best-fit calculation process of the tracking step327, mentioned above with reference toFIG.7. Coordinate vector322is used in the first iteration n=1 of the calculation process of tracking step327, while in the subsequent iterations n>1 values329nare used as calculated in the iteration immediately before.

In particular, normalization or conversion step325can have the structure described hereinafter, in the light of what has been said above about the way to calculate conversion parameters310. In a first calculation step325a, the ithcomponent B(k)i,nof magnetic induction vector at the kthsensor in the nthmeasurement, i.e. in the nthcollection cycle ofFIG.5, is calculated starting from the corresponding raw value Vj,nas:
Bi,n(k)=ΣjTi,j(k-opt)(Vj,n−Oj(k-opt))  [3]
whereOj(k-opt)is the offset vector, according to the three coordinate axes j=0 . . . 3 of the kthsensor, andTi,j(k-opt)is the matrix for correcting the errors due to gain variability of the kthsensor,
Oj(k-opt)and Ti,j(k-opt)being the values of Oj(k)and Ti,j(k), respectively, minimizing the amount [1] calculated in the step of computing the conversion parameters.

This passage makes it possible to remove the offsets and to make the magnetic field measurements isotropic. The normalization is advantageously completed by a further algebraic step consisting in applying the rotation matrix Rid to above obtained values B(k)i,n, the rotation matrix being calculated in a subsequent calculation step325bby minimizing the amount [2] and defined by three angles θ(k), φ(k), ψ(k).

In particular, in the case of the magnetoresistive sensors, the offset and the gain sensitively depend on the temperature. This means that the conversion parameters calculated at a given temperature are normally not suitable for normalizing magnetic field data read when the sensors have a different temperature. In order to assist the operator in the decision whether to ask the apparatus to carry out a step of computing new conversion parameters to be used for normalizing the magnetic field data in subsequent data collection and processing, in an exemplary embodiment, collection unit20comprises a sensor, not shown, for measuring the actual temperature at which magnetic field sensors22operate. Magnetic field sensor control unit25is configured to detect this actual temperature, for instance, when a data acquisition start command250is received, and before performing step of prearranging magnetic field sensors (FIG.5). Magnetic field sensor control unit25is also configured to calculate the difference between the actual temperature and the temperature at which the last calibration has been made and, if this difference is higher than a predetermined value, for emitting an alarm signal and, preferably, a request message asking whether to calculate new conversion parameter or not.

In the case of a system50according to the exemplary embodiment ofFIG.3, in which the data processing unit is implemented with such a device as a personal computer remote from data collection unit20, collection unit20advantageously comprises a housing28for an internal battery for electrically supplying sensor control unit25and magnetic field sensors22, in particular during the calibration step, in particular during the step of computing new conversion parameters. Advantageously, in order to limit battery consumption, sensor control unit25is configured to automatically block the electrical supply to collection unit20from the internal battery when a calibration stop command is received, in particular, after a predetermined delay time since after receiving the end-of-calibration signal.

For the sake of simplicity, reference has been made till now to the case in which magnetic field source10comprises or can be considered a single magnetic dipole10. However, in an exemplary embodiment, magnetic field source10can comprise a plurality of magnetic dipoles rigidly connected to each other or to body1so as to be differently oriented in the space. In this case, the position coordinates include:three spatial location coordinates of magnetic field source10with respect to array21of magnetic field sensors22;three orientation coordinates of magnetic field source10, andat least two orientation angles of array21of magnetic field sensors22with respect to geomagnetic field5.

Regardless the above, in another exemplary embodiment, as diagrammatically shown inFIG.9A, collection unit20can comprise, in addition to magnetic field sensors22, also a gravimetric sensor29, preferably mounted to support23. In this case, gravimetric sensor29makes it possible to know the orientation of the gravitational fieldgwith respect to support23and to array21of magnetic field sensors22. In this exemplary embodiment, the position coordinates include:three spatial location coordinates of magnetic field source10with respect to array21of magnetic field sensors22,at least two orientation coordinates of magnetic field source10, andthree coordinates of geomagnetic field5,
so that a complete information is provided, including three angles of array21with respect to the gravitational field6.

In an alternative exemplary embodiment, diagrammatically shown inFIG.9B, system50can comprise a device24to generate an auxiliary magnetic field7whose magnetic induction vectorB7has a direction that is different from the direction of magnetic induction vectorB6of geomagnetic field6, and a module B7that changes with time in a known way, for instance, it changes sinusoidally with time. The generator device can include, for instance, a pair of induction coils24aand24b, arranged opposite to each other with respect to support23and to the subject wearing the same.

In particular, in the not limitative example shown in Fig., auxiliary magnetic field7is a homogeneous field whose field lines are not parallel to the field lines of geomagnetic field6. In this case, in order to obtain a substantially homogeneous auxiliary magnetic field, induction coils24a,24bare parallel to each other and have a diameter larger than a minimum predetermined value.

In this exemplary embodiment, the position coordinates include:three spatial location coordinates of magnetic field source10with respect to array21of magnetic field sensors22,at least two orientation coordinates of magnetic field source10, andthree coordinates of geomagnetic field5,
so that a complete information is provided, including three angles of array21with respect to a system integral to the environment, and univocally determined with respect to the directions of geomagnetic field5and ofg, i.e. of geomagnetic field5and of auxiliary magnetic field7.

With reference toFIGS.10-11B, as an application of system50according to the invention, apparatuses100,200are described for tracking a subject's tongue1aand a subject's eyeball or eyeballs1b, respectively, in order to control such a peripheral device199,299as a robotic device and/or a communication device according to the movements of tongue1aor of eyeball/eyeballs1b. In these apparatuses, support23is configured to be fixedly worn on the subject's head9, in an appropriate position with respect to tongue1aor eyes1b, preferably in a lateral position with respect to the subject's head9. The magnetic field source is a small, preferably of cylindrical magnet10, configured to be fixed to tongue1a(FIG.10A) or to the subject's eyeball/eyeballs1b(FIG.11A), respectively. In the latter case, the size of magnet10does not preferably exceed Ø3 mm×1 mm, in order to be easily incorporated into a contact lens216(FIG.11B).

Apparatuses100,200include an interface and command device150between system50and peripheral device199. Interface and command device is configured to receive the position coordinates of magnetic field source10with respect to geomagnetic field5from data processing unit30of system50, and to interpret these parameters as a pointing direction of tongue1aor eyeball1btowards an icon163selected among the icons162displayed on a panel161arranged in front of the subject, each of which corresponding to a specific command for peripheral device199. Peripheral device199can also be a word processor device incorporated in the interface and command device. Preferably, interface and command device150is also configured to show the icon163selected by the subject, in order to confirm that the latter has selected the right command on panel161by the tongue or gaze direction.

In particular, interface and command unit150can be incorporated in a personal computer160or in an equivalent device, such as a tablet or a smartphone. More in particular, if data processing unit30of tracking system50is implemented with a personal computer or with an equivalent device, both data processing unit30of system50and interface and command unit150can be resident in a same computing machine160.

In particular, apparatus100can comprise a sound detector, not shown, connected to the interface and control unit150, which comprises a means for recognizing subject's verbal communication act, and is configured to discontinue the generation of control signals through panel161when subject's verbal communication is recognized.

With reference toFIG.12, as a further application of system50, a three-dimensional pointing apparatus400is described, in particular a three-dimensional writing device. The pointing apparatus400comprises a fixed three-dimensional drawing pad401, i.e. a drawing box401suitable for performing in 3D the functions performed in 2D by a conventional drawing pad, the three-dimensional drawing pad reproducing a drawing environment or command environment. The pointing apparatus400also comprises a pointing device1csuch as a pen a tip portion410of which is configured to be displaced in the three-dimensional drawing pad401, as well as a interface and command device450that, in particular, can be incorporated in a personal computer460or in an equivalent device. More in particular, if data processing unit30of tracking system50is implemented with a personal computer or with an equivalent device, both data processing unit30and interface and command device450can be resident in a same computing machine460. Interface and command device450is configured to display graphic elements on a display unit420, responsive to the position and/or to an active/inactive status of pointing device1cin the three-dimensional drawing pad401. Display unit420can comprise the monitor of personal computer460. In a preferred alternative, since the tracker generates a three-dimensional information, display unit420can comprise a VR viewer. In this case, magnetic field source10is preferably arranged at tip portion410of pointing device1c, and support23is integral to the three-dimensional drawing pad401, or even incorporated therein.

With reference toFIG.13, optionally, the three-dimensional pointing apparatus400can also comprise a shield element415made of a ferromagnetic material arranged for performing a relative movement with respect to a movement field of portion410of pointing device1cat which magnetic field source10Is fixed. This way, magnetic field source10can be shielded in a variable way with respect to sensors22. Therefore, tracking system50, through sensors22, can appreciate intensity changes of local magnetic field4, which can be associated to a variation of a display parameter or feature of a graphic element420. This feature can be, for instance, even in a selective way, a line thickness, a line or picture color or color tonality, a line or picture hatch density as shown in display unit420.

In an exemplary embodiment, not shown, the shield element is contained in the pen-like element, in particular in a tip portion of the pen-like element, and the pen-like element has a device for driving said movement accessible to the hand of a user.

With reference toFIG.14, as a further application of system50, an apparatus500is described for measuring a subject's graphometric features, in particular an apparatus for checking the authenticity of a subject's signature, or for investigating implications physiologic, and even pathologic aspects of some features of a subject's handwriting. Apparatus500comprises a pen-like element, for example a pen1d, and a surface501configured to feel the contact of pen tip502on surface501. Apparatus500also comprises an interface and command device550that, in particular, can be incorporated in a personal computer560or in an equivalent device. More in particular, if data processing unit30of tracking system50is implemented with a personal computer or with an equivalent device both data processing unit30and interface and command device550can be resident in a same computing machine560. Interface and command device550has a computing means that is associated to sensitive surface501for associating a plurality of graphometric features to a writing contact of pen-like element1don surface501.

In particular, magnetic field source10is arranged at a pen-like element portion510different from tip502, for example at an end portion of the pen-like element opposite to the tip portion, while support23, with the array of magnetic field sensors22, is configured to be fixed to the palm702of a hand3, for example by means of a belt23′. In this case, the graphometric features comprise two inclination angles of pen-like element1dduring the writing contact, and also the inclination of hand3in a reference system external to apparatus500. In an alternative, not shown, support23is integral to sensitive surface501, in which case the graphometric features comprise two inclination angles of pen-like element1dduring the writing contact.

In a further application, not shown, of system50, an apparatus for tracking displacements of a surgical tool1ein a patient's body, comprises the system according to the previous description, in which magnetic field source10is arranged at a portion of the surgical tool, in particular at a tip portion thereof, while support23, with array21of magnetic field sensors22, is configured to be fixed, for instance, to a surgeon's arm of, or to a support at the operating bed.

With reference toFIG.15, as a further application of system50, an apparatus for tracking displacements705of the fingers2of a subject's hand3, comprises at least one system according to the previous description, in which magnetic field source is arranged at a portion1fof a respective finger2of hand3, in particular at a phalanx1f. For instance, the magnetic field source, in the form of a magnet10, can be incorporated in a thimble701. Support23, with array21of magnetic field sensors, is configured to be fixed to the wrist702, as shown in the figure or, in an alternative not shown, to the corresponding subject's forearm, in both cases, for example, through a bracelet23′. An interface and command device750can be provided for receiving the tracking data of fingers2, and can comprise data processing units30of respective devices50for tracking respective fingers2of hand3, connected to respective sensor control units25through respective connection cables25′.

The foregoing description of exemplary embodiments of the invention will so fully reveal the invention according to the conceptual point of view, that others, by applying current knowledge, will be able to modify and/or adapt for various applications such embodiment without further research and without parting from the invention and, accordingly, it is to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiments. The means and the materials to implement the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology that is employed herein is for the purpose of description and not of limitation.