THEREOF

Technical Field

The present invention concerns a position indicator for a digitiser tablet. The invention further concerns a method of controlling said device.

Prior Art

In the information technology field, use of so called digitiser apparatuses, i.e. apparatuses capable of converting data generated in analogue form into digital data that can be interpreted by an electronic processor, is widespread. Among such digitiser apparatuses, digitiser tablets can be mentioned. Digitiser tablets are arranged to receive a data input in analogue form through a skimming and a compression of the tablet surface and to convert said data into a set of digital values, typically binary codes, which can be interpreted by a machine on which suitable software runs.

Digitiser tablets are currently used in several applications, for instance for affixing a user's signature on occasion of bank transactions.

In practice, in the above example, a user performs such an operation by drawing his/her signature on the tablet surface by means of a suitable writing instrument, commonly referred to as a digitiser pen. This is substantially a digital writing instrument comprising a position indicator, i.e. a device arranged to supply the software running on the machine with the geographical coordinates progressively intercepted by the tip of the indicator device on the tablet surface.

To this aim, the tablets are equipped with a matrix of antennas or sensors arranged to modify an electrical parameter, typically amplitude and frequency, in a signal generated or modified by them. Generally the antennas are excited by a known signal. Moreover, the antennas are equipped with circuits capable of modifying a parameter of an electrical signal when the tip of the writing instrument approaches or contacts the digitiser surface. A microcontroller processes the signals reflected by the antennas in search of possible alterations in the parameters consisting of amplitude and/or frequency because of the interaction with a position indicator that is in contact with or in proximity of the antennas. By associating the alterations possibly detected with the antennas from which the signal carrying such alterations comes, the microcontroller is capable of determining the position coordinates of the position indicator relative to the writing surface of the digitiser tablet.

Digitiser apparatuses must be capable of detecting the presence of the writing instrument even when the latter is not yet in contact with the digitiser tablet, but is in proximity of the antennas. The actual "touch" of the tip of the writing instrument onto the surface of the digitiser tablet corresponds to a "click" of the mouse according to the conventional technique in information technology and gives rise to a value of the pressure exerted by the writing instrument onto the surface of the digitiser tablet.

The proper and optimum operation of such digitiser apparatuses generally also requires communicating a value representative of the pressure exerted by the user onto the writing instrument and, consequently, exerted by the latter onto the surface of the digitiser tablet through said instrument, to the software running on the machine connected to the tablet.

Thus, the circuits employed in digitiser tablets are generally capable of modifying a parameter of an electrical signal depending on the pressure exerted by the tip onto the surface of the digitiser tablet. The operation of such circuits is known to the skilled in the art and can be based for instance on the physical principle of electromagnetic induction.

The variation of the parameter of the electrical signal, e.g. the energy or amplitude thereof, depending on the contact or the exerted pressure, is of limited amount and generally it has to be amplified in order it can be correctly interpreted by the software running on the machine connected to the digitiser tablet. Moreover, the electrical signal produced by the tablet is to undergo an analogue-to-digital conversion in order it can be interpreted by the machine

The known writing instruments for digitiser tablets are included into two main groups, depending on whether they require a power supply or not. In the first case, they are referred to as active devices and in the second case as passive devices. Active devices are generally capable of determining greater signal variations and hence of allowing a finer resolution of the measured values. Yet, active devices are more expensive, are highly power consuming and are complex to be manufactured. On the contrary, passive devices are free from the above drawbacks, since they do not require a power supply. Yet, such passive devices allow producing a smaller variation in the measured signal than the active devices and consequently they cannot attain the same precision. Passive devices are generally equipped with an L-C electrical circuit in which, by varying at least one of the parameters consisting of inductance and capacitance as the pressure exerted by the tip on the tablet varies, it is possible to obtain a variation of the oscillation frequency and consequently to modify a parameter in the electrical signal generated by the tablet with which the device comes into contact. In the prior art devices, the variation of the oscillation frequency is generally determined by the variation of the impedance of the L-C circuit the device is equipped with. In order to obtain such a variation, the device is therefore generally equipped with an axially slidable tip, associated with a variable inductor connected to the L-C circuit. An example of prior art device based on said principle is disclosed in EP 1 331 547 Bl. In the prior art devices, the movement of the slidable tip is generally opposed by resilient members arranged to bring the tip back to a rest condition when the device is moved away from the tablet and thus pressure is no longer exerted thereon.

The prior art devices generally include two resilient members consisting e.g. of corresponding helical springs. The two resilient members have a different compression resistance. A first spring, having a lower resistance to compression, determines the variation of the electrical quantity from the instant the tip of the device intercepts the surface of the digitiser tablet to the instant of intervention of the second spring, which opposes a higher resistance to compression. In this way, the first spring substantially meets the requirement of signalling that contact between the device and the surface of the digitiser has occurred, whereas the second spring meets the requirement of signalling the pressure exerted by the device on the surface of the digitiser tablet.

Yet, the provision of multiple resilient members or springs makes the device complex to be manufactured, and thus unavoidably expensive. Moreover, the provision of multiple resilient members makes the device more liable to the risk of performance deterioration and of variation in time of its electro-mechanical behaviour.

Some of those drawbacks are enhanced in that such devices are often used by different users and hence by different "hands". This is for instance the case in which the digitiser tablet and the writing instrument associated therewith are used in a bank or other facility in order to collect signatures of a plurality of users.

It is a first object of the present invention to provide a position indicator for a digitiser tablet, which is free form the drawbacks of the prior art- It is another object of the invention to provide a simplified and cheap indicator, which however enables attaining a high precision and which is reliable even after a prolonged use by different users.

It is a further object of the invention to provide an indicator of the above kind, which is simple to be manufactured and which therefore can be industrially manufactured with limited costs in great amounts.

It is a further, but not the last object of the invention to provide a method of controlling a position indicator, which method allows quickly and reliably recognising the spatial coordinates of the indicator relative to the digitiser tablet and the pressure exerted on the latter, even when such an indicator has a single resilient member.

Description of the invention

The above and other objects are achieved by means of the position indicator

("device") for a digitiser tablet and the control method therefor, as claimed in the appended claims.

The device according to the invention is capable of interacting with the digitiser tablet in order to modify at least one parameter, for instance amplitude and frequency, of an electrical signal generated by a circuit provided in the tablet. Advantageously, according to the invention, such a result is achieved by means of a device incorporating a single resilient member. Advantageously, using a single resilient member considerably simplifies the construction of the device

Further advantages of the invention result from the use of a reduced number of components manufactured by moulding plastic materials. Moreover, the components are configured so that they can be easily assembled.

Always according to the invention, a method of controlling the device is advantageously provided, which is arranged to improve the response rate of the software interpreting the signals coming from the digitiser tablet and to attain high reading precisions.

The control method according to the invention separately analyses both spatial coordinates, first through an approximate reading, determining the approximate or coarse location of the position indicator relative to the antennas of the digitiser tablet, and then through a specific and precise reading of the antennas within the region identified in the first reading. A third reading determines the value of the pressure exerted by the device on the tablet.

More precisely, according to the invention, the control method includes the steps of:

- generating one or more known signals, adapted to excite in controlled manner and according a predetermined scheme one antenna out of a plurality of antennas of a digitiser tablet arranged to interact with a position indicator, said antennas including a set of antennas associated with the "x" coordinates and a set of antennas associated with the "y" coordinates on the Cartesian plane;

- analysing the response to said excitation signal in order to determine an approximate position of the position indicator relative to a writing surface of a digitiser tablet according to a known scheme;

- analysing the response to said excitation signal in order to determine a precise position of the position indicator relative to a writing surface of a digitiser tablet according to a known scheme;

- analysing the response to said excitation signal in order to determine the value of the pressure exerted by the indicator on said writing surface according to a known scheme. The control method according to the invention includes a first step of approximate search for the positioning coordinates of the tip of the position indicator relative to the antennas of the digitiser tablet. In such a first step, substantially of coarse search for the position, the values of the coordinates are obtained on the basis of the amplitudes of the signals coming from the antennas and processed by a microcontroller.

In accordance with the invention, such a first step aims at searching for the approximate position of the position indicator in the shortest possible time. For this reason, advantageously, according to the invention, the signals coming only from some antennas, e.g. every third antenna, are read. In other words, according to this example, the antennas at positions 1, 4, 7, 10 ... are read, whereas the antennas at positions 2, 3, 5, 6, 8, 9 ... are not read. In order to further speed up the search for the approximate position of the tip of the position indicator, the antennas are excited with short excitation signals ("short bursts"), i.e. signals shorter than it would be required for a precise and optimum measurement of the coordinates. According to the invention, indeed, this first step only aims at detecting the presence of the position indicator, whereas the precise position will be searched for in a subsequent step.

In accordance with a preferred embodiment of the invention, first the signals coming from the vertical antennas associated with the coordinates located on the "x" axis are read, and then the horizontal antennas associated with the coordinates located on the "y" axis are read. Moreover, a single reading for each antenna is performed. Reading takes place by sending an excitation signal to the antenna and by consequently reading the reflected signal. The excitation duration and the read time will be short ("short burst and read time") and, at any rate, shorter than the duration and time used in the subsequent step of precise search for the position.

This step of scanning the signals reflected by the antennas is followed by the search for the actual position of the tip, which is carried out by comparing the energy values, i.e. the amplitude values, of the signals received from the antennas and by identifying the two antennas with the highest values belonging to the set associated with the "x" axis and the "y" axis, respectively.

This first search step ends with the comparison of the amplitude values of the signals from the antennas exhibiting the highest values with a predetermined threshold. If the amplitude values measured do not exceed the predetermined threshold, the first step is repeated in order to search again for the presence of the position indicator.

The method according to the invention includes a second step of precise search for the positioning coordinates of the tip of the position indicator relative to the antennas of the digitiser tablet. The values of the coordinates in such a second step are obtained on the basis of the amplitudes of the signals coming from the antennas and processed by the microcontroller.

This second step aims at precisely determining the exact position of the pen in plane "x, y". For this reason, the duration of the excitations signals for the antennas and the read times of the reflected signals will be longer ("long burst and read time") than in the first step of approximate search. The individual antennas are read several times in order to improve the precision of the result.

In this second step, the antennas are read as follows. Assuming that in the first step the tip has been approximately located on the antennas having coordinates "Xm" and "Yn", the signals of the adjacent antennas, preferably of the two antennas preceding and following the antennas with position "m, n", are read. Moreover, the signals from the antennas with position "m, n" are read again. Thus, the signals of antennas "Xm-2", "Xm- 1 ", "Xm", "Xm+1 ", "Xm+2" are read, where "Xm" is the antenna with the highest signal amplitude value found during the first step of approximate search. The process is repeated for axis "y", by reading antennas "Yn-2", "Yn-l ", "Yn", "Yn+l ", "Yn+2", where "Yn" is the antenna with the highest signal amplitude value found during the first step of approximate search. Moreover, the signals of the antennas are read several times, e.g. five times, in order to improve the precision degree and eliminate read errors due to background flicker and to imprecisions.

In this second step, the amplitude values of the signals are processed according to a polynomial interpolation algorithm. In this way, a bell-shaped curve is built from the obtained values and its actual maximum, corresponding to the exact position of a coordinate, for instance on the "x" axis, is computed. The same procedure is carried out for the second coordinate, for instance on the "y" axis. The final "x" and "y" coordinates will be an average of the results obtained from the readings, e.g. five readings.

In a third step of the method according to the invention, the pressure exerted by the tip of the position indicator on the writing surface, i.e. the surface associated with the antennas of the digitiser tablet, is measured.

The pressure values for such a third step are obtained on the basis of the frequencies of the signals coming from the antennas and processed by the microcontroller.

This third step aims at determining whether the pen is in contact with the writing surface of the digitiser tablet and the actual value of the pressure exerted on said surface. In order to increase the speed of the search for the pressure value, this step is performed for the antennas of a single coordinate, for instance on the "x" axis or the "y" axis. Actually, it has been realised that the other coordinate provides redundant information and thus it does not contribute to the measurement precision.

In this third step, the signal coming from the antenna having provided the highest amplitude value in the second step is processed several times, in the manner described below.

The third step includes a step of searching for the phase offset between the antenna excitation signal and the reflected signal processed by the microcontroller. More precisely, two peaks are determined in the curve of the reflected signals and their frequencies are then calculated. The algorithm then determines whether the pen is in contact with the surface of the digitiser depending on the phase offset measured between the frequency of the reflected signal and the known frequency of the excitation signal.

The pressure exerted by the tip of the position indicator onto the surface of the digitiser tablet is proportional to the phase shift between the reflected signal and the excitation signal.

Through a dedicated electronics, such a shift is made to correspond to an analogue value that, once measured, allows directly obtaining the actual pressure value. In order to increase the reliability and precision degree, the measurement is performed several times.

The control method according to the invention further includes some optimisation functions.

A first optimisation function concerns blocking the position skip while the position indicator is always present. In order to prevent position skips from the right to the left and vice versa, i.e. false positives, which are physically impossible, a check about the last detected position is made. For instance, if the previous position corresponds to the antenna with coordinates "X5" and the newly measured position corresponds to the antenna with coordinates "X31", the value of the new position is discarded.

A second optimisation function concerns blocking the position skip when the pen leaves the sensitivity area of the digitiser tablet. According to such a function, when the position indicator leaves the useful measurement area of the digitiser tablet and re-enters at another position, the optimisation function performs a position scanning step, by resetting (i.e. setting to zero) the values corresponding to the last reading.

A third optimisation function concerns reducing the effects of the trembling. To this end, the function has a "sliding window" system based on the last detected values. In this manner, the linearity of the stroke drawn by the tip of the writing instrument is increased, while preserving the data transmission rate.

A fourth optimisation function concerns reducing the "false positives" in order to prevent false coordinates from being transmitted while the touch is activated. More particularly, when the tip is very close to the tablet surface, a number of readings is waited for before activating the "touch" of the position indicator.

More generally, the device and the method according to the invention allow significantly reducing the occurrence of false signals indicating the presence of the device in writing position on the digitiser tablet, such signals being for instance due to the trembling of the hand of the user holding the device or to the writing device being made to approach the plane of the surface of the digitiser tablet in an oblique direction.

The control method according to the invention advantageously enables overcoming the mechanical limit due to the presence of a single resilient member, preferably having an elastic module which is substantially constant as the length of the resilient member varies. Brief description of the figures

Some preferred embodiments of the invention will be given by way of non-limiting example with reference to the accompanying drawings, in which:

- Fig. 1 is a plan side view of the position indicator for a digitiser tablet according to a preferred embodiment of the invention;

- Fig. 2 is a sectional view, taken along plane A - A, of the device shown in Fig. 1 ;

- Fig. 3 is an exploded view of the device shown in Fig. 1 ;

- Figs. 4A to 4C are flow charts of as many procedures of the control method according to a preferred embodiment of the invention;

- Fig. 5 is a schematic view of the spatial arrangements of the antennas in a digitiser tablet.

Detailed description of some preferred embodiments of the invention

Referring to Figs. 1 to 3, there is shown a position indicator for a digitiser tablet, made in accordance with the invention and generally indicated by reference numeral 11. Indicator 11 substantially includes a casing 13 made by a corresponding hollow tubular cylindrical body. A first front portion 15 and a second rear portion 17 are defined in casing 13. Moreover, casing 13 has an opening 19 at its forward end. Preferably, the forward end of the first portion 15 of the tubular body of casing 13 is tapered.

A frame 21 is housed in casing 13. Preferably, casing 13 and frame 21 are made of plastics. Always in accordance with a preferred embodiment of the invention, both portions 15, 17 of casing 13 are joined together by hot welding in order to create a single body having the required solidity. In the example illustrated, the rear end of the second portion 17 of the tubular body of casing 13 has an opening 17a, which preferably is closed by a cap.

A first axial front seat 23 directed towards opening 19 of casing 13, and a second side seat 25, directed towards internal wall 27 of casing 13, are defined in frame 21.

A resilient member 29 is provided in the first axial front seat 23. In the example illustrated, resilient member 29 includes a helical spring. Indicator 1 1 further comprises a tip 33 partially projecting outwards through opening 19. A slide 34 made of ferrite is provided between tip 33 and resilient member 29. Slide 34 is arranged to transmit the axial movement of tip 33 to resilient member 29 housed in axial seat 23 of frame 21. Said axial movement of tip 33 occurs when an axial compression is exerted on tip 33, e.g. when indicator 11 is brought with tip 33 in contact with the writing surface of a digitiser tablet.

A sleeve 35 surrounding ferrite slide 34 is further housed in the tubular body of casing 13. A wire 37 of electrically conducting material is wound around sleeve 35. A printed circuit 39 is housed in the second side seat 25 of frame 21. Said printed circuit 39 incorporates an electronic circuit to which both ends 37a, 37b of wire 37 of conducting material wound around sleeve 35 are connected. In the example illustrated, sleeve 35 substantially includes a bobbin having a central core and two disc-shaped wings arranged at the core ends, preventing wire 37 of conducting material from slipping off from the bobbin core.

In accordance with the preferred embodiment of the invention illustrated here, casing 13 internally has a radial projection 41. In the example illustrated, said projection 41 is located at about half length of rear portion 17 of casing 13. Correspondingly, frame 21 has a cantilevered rear base 43 arranged to cooperate with radial projection 41 of casing 13 in order to keep said frame 21 associated in stable way with casing 13. At its forward end, frame 21 has a base 45 having a recess 47 in which the disc-shaped rear wing of sleeve 35 engages. Always referring to the example illustrated, tip 33 has a front portion 33a passing through opening 19 of front portion 15 of the tubular body of casing 13, and a rear portion 33b, with greater cross-sectional size, slidably mounted in an axial cylindrical seat 49 provided in the tubular body, behind opening 19 for tip 33, and formed in the wall of said tubular body.

According to the invention, slide 34 forms the core of a variable inductor defined by slide 34 and wire 37 wound around sleeve 35. Slide 34, being axially slidable in sleeve 35, causes the variation of the oscillation frequency of the L-C circuit the variable inductor is associated with. Sliding of slide 34 is limited in a backward direction by resilient member 29 abutting, with its end opposite the end in contact with slide 34, against the bottom of axial seat 23, and, in the other, forward direction, by tip 33.

Tip 33, ferrite slide 34, sleeve 35 and resilient member 29 are to be assembled so as to ensure the optimum axial alignment and the substantial lack of clearance between the parts, in order to ensure the proper operation and the necessary fluidity of movement during writing.

Referring now to Figs. 4A, 4B, 4C and 5, the steps of the method of controlling a position indicator for a digitiser tablet according to the invention will be described.

Referring in particular to Fig. 5, two sets of antennas, each comprising four antennas 71, 73 associated with "x" axis and "y" axis, respectively, are schematically shown. The antennas illustrated are representative of part of the antennas provided in a conventional digitiser tablet of a kind known per se to the skilled in the art, so that it will not be disclosed in detail. Each antenna 71, 73 is identified on the writing plane of the digitiser tablet by a corresponding spatial coordinate "XI", "X2", "X3", "X4" and "ΥΓ, "Y2", "Y3", "Y4".

Referring to Fig. 4A, step 101 is the "start" step for the process of detection of the presence of position indicator 1 1 in writing position and of the pressure possibly exerted by the indicator onto the writing surface of the digitiser tablet. By writing position of indicator 11 a configuration of indicator 11 relative to the digitiser tablet is meant, in which tip 33 is near or in contact with the writing surface of the digitiser tablet. Said writing surface of the digitiser tablet is associated with antennas 71, 73. It is clearly apparent that, when tip 33 is in contact with the writing surface of the digitiser tablet, the tip will apply a certain pressure onto the tablet.

At step 103, the scanning routine for the approximate detection of the presence of indicator 11 is indicated, which routine will be described in more detail with reference to Fig. 4B.

If, at step 105, the approximate position scanning routine has produced a signal indicating the presence of indicator 11, the precision scanning routine is performed at step 107 in order to identify the coordinates on the "x" and "y" axes. Such a routine will be described in more detail with reference to Fig. 4C. At step 109, the routine for detecting the pressure with which indicator 1 1 is pressed onto the writing surface of the digitiser tablet is performed, which routine will be described in more detail below. If, on the contrary, at step 105, the approximate position scanning routine did not produce a signal indicating the presence of indicator 11, the approximate position scanning routine is performed again at step 103.

If, at step 109, the precision scanning routine has produced a signal indicating the pressure exerted by indicator 1 1 onto the writing surface of the digitiser tablet, at step 111 the frequency of the signal coming from the antenna having generated the signal with the greatest amplitude in the precision scanning routine is compared with a predetermined frequency corresponding to the antenna excitation signal. If, on the contrary, at step 109, the precision scanning routine has produced a signal that is not representative of the pressure exerted by indicator 11 onto the writing surface of the digitiser tablet, a condition of "presence without touch" of tip 33 occurs, at step 1 13 only the data concerning the coordinates on the "x" and "y" axes are collected, and the control is assigned again to step 105.

At step 115, if the frequency of the signal reflected by the antenna having generated a reflected signal with the greatest amplitude in the precision scanning routine has exceeded the predetermined threshold of the comparison of step 111, i.e. the touch of the tip of indicator 11 onto the writing surface of the digitiser tablet has been detected, the pressure applied by indicator 11 onto the writing surface of the digitiser tablet is measured. Said measurement is carried out by comparing several readings of a single antenna, defined in optimum manner, which are submitted to convolution and compared with the phase shift relative to a known signal.

At step 117, position coordinates "X, Y" of indicator 11 that have been identified and pressure value "P" are transmitted to a processing unit or electronic microcontroller. At the end of the process, the control is assigned again to step 105.

Referring to Fig. 4B, the approximate scanning routine for detecting the presence of indicator 11 in writing position is shown.

Said routine initially provides, at step 201, for using narrow time parameters ("short burst and read time") in order to optimise the time required for approximately locating indicator 11. At steps 205, 207, scanning of the antennas associated with coordinates "x" (XI ... X23) and coordinates "y" (Yl ... Y19) takes place sequentially, according to a predetermined scheme. For instance, scanning takes place preferably every second antenna: xl, x3, x5 ... and yl, y3, y5 ...

At step 209, the antenna having produced the signal with the greatest amplitude, i.e. the highest energy, is identified.

At step 211, the amplitude of the signal of the antenna with the highest energy is compared with a predetermined threshold. If at step 211 the amplitude of the signal of the antenna with the highest energy is lower than said predetermined threshold, the control is assigned to step 213 corresponding to a condition in which indicator 11 is not present, and hence to step 205. If on the contrary at step 211 the amplitude of the signal of the antenna with the highest energy exceeds said predetermined threshold, the control is assigned to step 215 corresponding to the condition of presence of indicator 11 in writing position. At the end of the routine, the control is assigned to step 107 in order to perform the precision scanning routine, allowing perfecting the identification of the position coordinates of indicator 11 on the writing surface of the digitiser tablet.

Referring to Fig. 4C, the precision scanning routine for detecting the presence of indicator 11 in writing position is shown.

Said routine initially provides, at step 301, for using extended time parameters

("long burst and read time") in order to optimise precision for precisely locating indicator 1 1. At step 303, the amplitude of the reflected signal coming from the antenna having generated the signal with the highest energy in the previous step and the amplitudes coming from the four antennas around the antenna having generated the signal with the highest energy on each coordinate axis in the plane are read a number of consecutive times. If Xm and Yn are the antennas having generated the signal with the highest energy for the respective coordinate, there will be analysed the amplitudes of antennas Xm-2, Xm-1 , Xm, Xm+1, Xm+2 in order to determine the coordinate on the "x" axis (step 303), as well as the amplitudes of antennas Ym-2, Ym-1, Ym, Ym+1, Ym+2 in order to determine the coordinate on the "y" axis (step 305).

The amplitude (i.e. energy) values obtained define a Gaussian curve, the mathematical maximum of which corresponds to the coordinates that are used in order to determine the position of writing device 11. At the end of the routine, the control is assigned to step 109. The third step of the method according to the invention will now be described, in which step the pressure exerted by tip 33 of position indicator 1 1 on the writing surface of the digitiser tablet, i.e. the surface associated with antennas 71 , 73, is measured.

The pressure values for such a third step are obtained on the basis of the frequencies of the signals coming from antennas 71, 73 and processed by the microcontroller.

This third step aims at determining whether the pen is in contact with the writing surface of the digitiser tablet and the actual value of the pressure exerted on said surface. In order to increase the speed of the search for the pressure value, this step is performed for the antennas of a single coordinate, for instance on the "x" axis or the "y" axis. Actually, it has been realised that the other coordinate provides redundant information and thus it does not contribute to the measurement precision.

In this third step, the signal coming from the antenna having provided the highest amplitude value in the second step is processed several times, in the manner described below.

The third step includes a step of searching for the phase offset between the excitation signal of the antenna having provided the highest amplitude value in the second step and the reflected signal processed by the microcontroller. More precisely, two peaks are determined in the curve of the reflected signal and their frequencies are then calculated. The routine determines whether indicator 11 is in contact with the surface of the digitiser on the basis of the phase offset measured between the frequency of the reflected signal and the known frequency of the excitation signal.

The pressure exerted by the tip of the position indicator onto the surface of the digitiser tablet is proportional to the phase shift between the reflected signal and the excitation signal.

In a preferred embodiment of the invention, a specific and dedicated electronic unit allows allotting an analogue value to such a phase shift, which value, once it has been measured, allows directly obtaining the actual pressure value. In order to increase the reliability and precision degree, the measurement is performed several times.

Advantageously, thanks to such a control method, a considerable signal processing rate and a quick response are obtained. Moreover, the aforesaid method allows eliminating false positive signals corresponding to configurations in which the writing device is not in writing position.