FR2607289A1 - Device of the graphics tablet type for converting a drawing into data representing the said drawing - Google Patents

Abstract

The invention relates to a device of the graphics tablet type for converting a drawing or the like on a surface into data representing the said drawing, for example for the purposes of processing by calculating means, characterised in that it comprises: - a resistive surface 10 of general rectangular shape on which the said drawing is to be made with the aid of a stylus 30, - two pairs of linear electrodes 18a, 18b, 20a, 20b arranged at the periphery of the said surface, the two electrodes of a single pair being essentially parallel and the ends of the electrodes being electrically linked together at the four corners of the said surface, each electrode being constituted by the series mounting of a plurality of individual resistors 16 whose common points are linked electrically to the associated edge of the said surface at equidistant points, - commutation and electrical power supply means 26, 38 for linking together, periodically and alternately between the two pairs, the two ends of each electrode of the same pair and for supplying the said electrodes with power at a given voltage; the two pairs of electrodes being thus alternately activated; and - means for picking off, on each half-cycle, the potential at the contact point P between the stylus 30 and the surface 10, the two potentials picked up in the course of two successive half-cycles being representative of the instantaneous coordinates of the said point in a reference frame defined by the position of the said electrodes.

Description

 The present invention relates generally to devices of the graphics tablet type, that is to say generally to devices for converting graphic traces on a medium into information, for example digital information capable of to be processed by computing means.

 We already contact in the art a number of graphics tablets, which use several principles.

 We distinguish tablets that use a magnetic principle, and those that use an electric principle. It is to the latter that the invention relates.

 A first category of known electric graphic tablet comprises a mesh which defines a matrix of points. The detection of an instantaneous point of the trace is carried out by short-circuit, capacitive effect or even hall effect, and the coordinates of said point are in the form of an activated row-column pair. However, this device is disadvantageous in that the precision of such a device is limited by the density of the mesh that can be produced in practice, and in that the increase in this density leads to a significant additional cost.

A second category of known graphic tablet operating according to an electrical principle consists in providing two spaced parallel resistive layers each comprising a pair of electrodes, the two pairs being capable of establishing two orthogonal electric fields. Pressing a stylus on the writing surface brings the two layers into local contact. While one pair of electrodes is activated, the other two electrodes are connected together and collect the voltage at the contact point, which is a function of the position of said point in the direction perpendicular to the active electrodes. The roles of the two pairs of electrodes are then swapped and the position of the point in
orthogonal direction is obtained. We thus obtain
periodically a couple of coordinates (x, y), the set
of these couples being representative of the layout.

However, such a system has the drawback
deny being relatively lumpy, especially by the need
sity to arrange two narrowly spaced resistive layers.

The present invention aims to overcome the in
agree with the prior art and propose a
device that is low cost and has good
n performance in terms of both precision and speed.

To this end, the invention provides a device of the graphic tablet type for converting a plot or the like on a surface into data representative of said plot, for example for the purposes of processing by calculation means, characterized in that it comprises:
- a resistive surface of generally rectangular shape, on which said tracing must be carried out using a stylus,
- two pairs of rectilinear electrodes arranged around said surface, the two electrodes of the same pair being essentially parallel and the ends of the electrodes being electrically connected to each other at the four corners of said surface, each electrode being formed by mounting in series of a plurality of individual resistances whose co-ordinate points are connected elec triquenent to the associated edge of said surface at equidistant points,
- cammitation and electrical supply means for connecting together, periodically and alternately between the two pairs, the two ends of each electrode of the same pair and for supplying said electrodes under a given voltage; the two pairs of electrodes being thus altereativeent activated;
- Means for taking, at each alternanoe, the potential at the point of contact between the stylus and the surface, the two potentials taking off during the two successive alternnoes being representative of the instantaneous coordinates of said point in a frame defined by the position of said electrodes.

The invention will be better understood on reading the following detailed description of preferred embodiments thereof, given by way of example and made with reference to the accompanying drawings, in which
FIG. 1 is a schematic perspective view of the essential elements of a graphics tablet according to the present invention,
FIGS. 2a and 2b are schematic front views illustrating two states of the tablet of FIG. 1,
FIG. 3 is a cross-sectional view of an alternative embodiment of the physical structure of the tablet of FIG. 1,
FIG. 4 is a schematic representation of a first variant embodiment of the invention,
FIGS. 5a and 5b illustrate two states of the graphics tablet of FIG. 4, and
- Figure 6 is a schematic representation of another alternative embodiment of the invention.

 Referring first to Figure 1, a graphics tablet according to the invention comprises a surface 10 having a homogeneous electrical resistivity, which will advantageously be supported by a rigid counter-plate (not shown).

 The sheet 10 is here rectangular and dead two large lateral edges 12a, 12b and two small edges 14a, 14b, respectively upper and lower.

 On each of the edges is arranged an electrode which, in accordance with the invention, is constituted by the series connection of a set of electrical resistors 16. The respective electrodes of the edges 12a, 12b and 14a, 14b are referenced 18a, 18b and 20a, 20b.

 As can be seen in FIG. 1, the common points between the successive resistances of an electrode are brought into electrical contact with points regularly spaced from the associated edge of the surface 10 and extending over the entire length of said edge. . The electrical contact can be ensured by any appropriate means, and in particular mechanical means forming a clamp for pressing the end wires of the resistors against the surface.

 The tablet further comprises four switches 22a, 22b, 24a, 24b, the two terminals of which are mounted at the ends of the respective electrodes (that is to say at the corners of the surface), as shown.

 These switches, as will be seen in detail below, may be of any suitable type so that they can be controlled in voltage or in current at a rapid rate. These may include electromagnetic relays, integrated switches made using so-called C-MOS technology, transistors mounted as switches, etc.

 A control unit 26 delivers to each of said switches, along the dashed lines, the appropriate control signals, being clocked by a clock 28.

 There is also a writing tip or stylus 30 whose conductive writing end 30a is connected by a flexible wire 32 to the input of an analog / digital converter 34. This converter delivers as an output, under the control of the control unit 26. digital signals which are applied to a central unit 36 or the like, for example that of a microcomputer, for further processing. The signal generated by the clock 28 is also transmitted to the central unit 36 for synchronization purposes.

 Finally, a DC voltage generator 38 applies a positive voltage + V to the connection point between the electrodes 18b and 20b, while the connection point between the electrodes 18a and 20a is connected to ground (O volt).

 It should be noted that the four electrodes 18a, 18b, 20a, 20b are electrically connected to each other at the four corners of the surface 10, to form a string of resistors closed on itself.

 The device schematically described above operates in the following manner. Under the control of the clock 28, the control unit 26 sends to the switches 22a, 22b and 24a, 24b control signals so as to result in two situations. The first situation (phase x) consists in closing the switches 22a, 22b leaving the switches 24a, 24b open.

 Thus, the two ends of the electrode 18a are short-circuited by the switch 22a. These two ends are at the same potential (ground) and therefore no current flows in the electrode. As a result, all the contact points of the resistors considered with the surface 10 are at the potential Ov, and a continuous and rectilinear electrode is thus produced as a first approximation along the edge 12a of the sheet 10.

 By analogous reasoning, the fact of short-circuiting the opposite ends of the electrode 18b makes it possible to produce an almost continuous electrode at the potential + V.

 With regard to the other electrodes 20a, 20b, by ensuring a regular voltage drop over their entire length, they contribute to creating in the region associated edges of the equipotential sheet situated in the extension of those created by the electrodes 18a and 18b.

 It is understood that the latter electrodes thus create in the sheet 10, relatively halogen and provided with a determined resistivity, a horizontal electric field Ex which is essentially uniform.

 Thus, if we apply the conductive tip 30a of the stylus 30 at a point P on the surface 10, this tip will be brought to a potential which will be between Ov and + V and which will depend on the distance counted horizontally between the point P and for example the electrode 18a, (the distance Q between the electrodes 18a and 18b being well determined).

The homogeneity of the electric field Ex in the plane of the surface 10 leads to a linear relationship between this potential and this distance, a relationship which is expressed by
Vx = V.- xp
l
The value of the voltage Vx collected on line 32 (relative to the mass) thus makes it possible to determine the coordinate xp of the point P where the tip 30a of the stylus is located with respect to the reference xOy as represented in FIG. 2a. that this value can be exploited by the central unit 36 for the purposes of memorization, display on screen, printing, etc., the analog-digital converter 34, under the control of the control unit 26, transforms the voltage Vx, comprised between Ov and + V, in a binary word of N bits, for example in an eight bit byte, which is then transmitted to the central unit 36 via an input port / outlet (not shown).

 It can be noted here that the conversion of the voltage Vx into an eight-bit word makes it possible to obtain 28, 256 values of the abscissa xp, ie an absolute precision of the order of 0.4%.

 Once the abscissa byte has been determined and loaded, these operations being clocked by the clock signal 28 or a signal directly obtained from it, the control unit 26 operates so as to pass in phase y, in which the switches 24a and 24b become closed, while the switches 22a and 22b open.

As a result, by reasoning similar to that which has been set out above and which will not be reproduced, in order to avoid adding to the description, the electrodes 20a and 20b are activated to the detriment of the electrodes 18a and 18b ( Figure 2b), and that a representative voltage Vy
V.Yp of the ordinate yp of the point P according to the relation Vy = L is available on the stylus and converted by the converter
A / D 34 into a corresponding numerical value.

 As for phase x, these operations are clocked by the control unit 26 to transmit to the central unit 36 an eight bit word representative of said ordinate.

 The control unit 26 will act to switch alternately from the "x" phase to the "y" phase at any suitable frequency, for example of the order of 100 to 1000 switches per second.

 Of course, any variant or modification can be made to this basic embodiment. In particular, it will be possible, in particular for the purpose of obtaining the same resolution in x and in y, to use a surface 10 of square shape, ie L = Q. In addition, to increase the resolution, the abscissa and ordinate information may be coded into longer digital words, for example 12 or 16 bits, in order to increase the resolution of the digitization. Furthermore, the reference O of the reference xOy may be slightly shifted inward relative to the upper left corner of the surface 10, in order to avoid possible measurement errors due to the effects of the edges of the surface. Such an offset will advantageously be managed by associated software from the central unit 36.

 In the embodiment described above, the voltage Vx or Vy representative of the corresponding coordinate is routed from the tablet directly by the stylus 30 and its cable 32, and the stylus 30 must imperatively be in contact with the conductive surface 10.

 The variant embodiment of FIG. 3 makes it possible, within the framework of the invention, to use a free stylus, devoid of wire, and which can therefore be, for example, any desired writing instrument, and to have above the surface. conductive a sheet of renewable ordinary paper, the device being responsible for transmitting to the central unit 36 the digital information relating to what is written on this paper with the writing instrument.

 The tablet comprises a rigid base or support 40 made of an electrically insulating material, such as a molded plastic material.

 On the upper surface of the support 40 is fixed a conductive metal sheet 42. A set of elements 44 forming spacers, electrically insulated are fixed or otherwise attached to the support 40, the conductive sheet 42 being interrupted at their level.

These elements 44 can be arranged in etoelonne lines, staggered, etc., with a mutual spacing which can be of the order of 1 to 10 mm. They are intended, as shown in Figure 3 to maintain the resistive sheet 10 resting thereon normally spaced from the conductive sheet 42, for purposes explained below. Finally, a sheet of plain paper 46 is placed and possibly immobilized on the resistive sheet 10.

 When the point 30a of the stylus 30, this time free and of any type, is applied with a certain pressure against the sheet 46, to write something on the latter, then the ordinary sheet 46 and the resistive sheet 10 deform slightly downwards at the point of contact P, until the resistive sheet 10 comes into electrical contact with the conductive sheet 42. Then it is understood that the conductive sheet 42 is brought as a whole to the potential of the point P (which is representative from the abscissa or the ordinate of this point), and this voltage is taken from said sheet, for example via the conductive wire 48, to be sent to the analog / digital converter.

 Of course, the elements 44 will be designed so that, when the tip 30a of the stylus comes to apply the pressure immediately above one of them, this element shrinks or deforms enough to not obstruct the coming in mutual contact of the surfaces 10 and 42. The degree of rigidity of the surface 10 will also be decisive for the contacting to be correct.

 An insulating protective sheet (not shown) can be added to the above structure immediately above the resistive surface 10. Such a sheet will prevent premature deterioration of the sheet 10 and may also contribute, in combination with the latter, to the deformability qualities required for good contact with the conductive layer 42.

 In addition, it is understood that the structure of the tablet described above can be inverted, the conductive sheet being arranged above the resistive sheet.

 Finally, it can be indicated that a spacing of the order of a few tenths of a millimeter between the two sheets to be brought into contact will in most cases give satisfactory results.

 It should be noted that, in this variant, only the physical structure of the graphics tablet is modified, the electrical and electronic part (controlled switches, electrodes with resistance chains, etc.) being preserved here.

 It is understood that a determining element in the construction of the structure of FIG. 3 will reside in the nature and the behavior of the spacers 44. It may be any rigid element, elastically deformable or even elastically retractable, and in particular bosses produced in one piece with the base 40, pins attached to the latter, micro-balls made of elastic material, for example partially embedded in recesses in the base 40 to ensure that they are held in place in the plane of the shelf, small sliding rods drawn upwards by elastic means such as a membrane, internally hollow bosses, etc.

 In all cases, these elements 44 will be concluded to offer insufficient resistance to penetration under the pressure of the tip 30a, but sufficient to prevent the user's hand leaning on the tablet for writing purposes from causes an unexpected contact between the resistive sheet 10 and the conductive sheet 42.

 We will now describe with reference to Figure 4 an alternative embodiment of the electronic part of the graphics tablet according to the invention. In this figure, the elements or parts identical or similar to those of Figure 1 are designated by the same reference numbers.

 The tablet comprises a resistive sheet 10 bordered by four electrodes 18a, 18b, 20a 20b, formed as described above. Two first diodes 50a, 50b are mounted in parallel with the electrodes 18a, 18b so as to be conductive from the top to the bottom. Two other diodes 50a, 50b, are mounted in parallel with the electrodes 20a, 20b, being conductive from the right to the left of the figure.

 The central unit 36 controls a square voltage generator 54, one first terminal of which is connected to the common point between the electrodes 18b and 20b and the other terminal (ground) of which is connected to the common point between the electrodes 18a and 20a, both above points being thus diagonally opposite.

 The generator 54 is capable of delivering a square wave voltage of peak values - V and + V, V being able to be equal for example to 5 volts.

 The conductive tip 30a of the stylus 30 is connected to the input of a circuit 56 of "absolute value" function.

 The output of the circuit 56 is connected to the input of the analog / digital converter 34, the output of which, serial or parallel as the case may be, is directed towards the central unit 36.

 A delay circuit 58 receives from the central unit 36 a timing signal which also controls the generator 54. The delay circuit 58 is intended to control the instants of conversion by the converter 34 as a function of the output state of the signal. - rator 54 and therefore of the state of the tablet.

The device described above operates as follows
- when, under the control of the central unit 36, a voltage of + 5 volts is applied to the common point between the electrodes 18b and 20b, all the other points of the surface 10 / electrodes assembly are at a potential era or lower at 5 volts, but greater than or equal to b O volt (mass). As a result, the diodes 52a and 52b are conductive, while the diodes 50a and 50b are blocked. As shown in FIG. 5b, the two electrodes 20a and 20b are thus activated (at low voltage drops near in the diodes 52a and 52b).

It is thus possible to measure, as described above, the voltage present at the point of contact P, which is representative of the ordinate of said point;
- Conversely, when the generator 54 delivers a voltage of - 5 volts, it could be demonstrated by analogous reasoning that it is the electrodes 18a and 18b are activated. The voltage Vx available at the stylus output is representative of the abscissa of the point
P, but this voltage is negative, this is why circuit 56 has been provided, the role of which is to convert this negative value into the positive value of the same amplitude, compatible with the input of the converter 34. The corresponding situation is illustrated in Figure 5a.

 It has been indicated in FIGS. 5a and 5b, the voltage drops of approximately 0.3 volts in the activated electrodes, corresponding to the conventional voltage drop in a germanium diode. In a first case, the measurement errors of the voltages Vx and Vy which result therefrom can be compensated by calculations in the associated control software or by hardware modification of the tablet.

 In another case, the four diodes 50a and 50b, 52a, 52b could be replaced by equivalent circuits (for example b operational amplifiers) providing a substantially zero voltage drop.

 FIG. 6 represents yet another variant of the electronic circuit of a graphic tablet of the invention.

 There is the resistive surface 10, the four electrodes 18a, 18b, 20a and 20b, as well as the four diodes (or equivalent) 50a, 50b, 52a, 52b.

 There is also the generator 54, controlled by the central unit 36, a first output terminal of which is applied to the common point between the electrodes 20a and 18b, and the other terminal of which is connected to the diagonally opposite point. These two points leave respectively two additional diodes 60, 62 whose other terminals are connected to two analog / digital converters 34a and 34b, respectively.

The voltage present at the point of contact P between the surface 10 and the stylus 30 is forced to O volt (input mass of the two converters 34a, 34bol
As in the embodiment of FIG. 4, the sign of the output voltage of the generator 54 will selectively activate one or the other of the pairs of electrodes 18a, 18b and 20a, 20b For example, when the said feren that of the potential between the upper right and lower left corners of the sheet 10 is positive, the diodes 52a and 52-b are conductive and the electrodes 20a and 20b are activated, but are at floating potentials relative to the reference voltage ( 0 volts) present at point P. Thus, when the point P of the upper electrode 20a is moved by movement of the stylus 30, the difference in potential
between the two will decrease in a linear relationship
with this displacement. It’s that difference of
potential which is collected at the input of the converter
34b, and we thus have a measure of the ordinate of the point P.

A similar reasoning can be made
for the measurement of the abscissa.

Regarding the respective values
the resistivity of sheet 10 and the resistance
overall electrodes (number of individual resistors.
16 x ohmic value of these resistors), effective tests
killed by the plaintiff have shown that satisfactory results
doing were obtained with sheet resistivity
in the range of 2 to 10 ohms and with overall resistance
of each electrode in the range of 50 to 500 ohms. In this way, the individual resistances 16 of the non-activated electrodes, of a value much lower than the resistivity of the sheet, impose regular spacing of the equipotentials at the corresponding limits of said sheet, in order to greatly attenuate the edge effects.

 To obtain precise results, a large number of individual resistors 16 in the same electrode is however not essential. We can for example be satisfied with 20 to 60 resistors on an edge of a few tens of centimeters long. It is however important, for a regular profile of the equipotentials as mentioned above, that the ohmic values of the resistors 16 are mutually equal with good precision and that the points of contact of its resistors with the surface 10 are as much as possible equidistant, with a pitch for example of the order of 5 to 10 mm.

 Furthermore, and in particular in the case where the resistive surface is not square, it will be possible to provide resistance values and different steps on the horizontal edges and the vertical edges.

 In addition, if possible edge effects lead to undesirable deformation of the equipotentials in the corners of the sheet, it will be possible to provide in these regions resistances with non-regular values and / or steps to compensate for these defects.

 Of course, the present invention is in no way limited to the embodiments described above and shown in the drawings, and a person skilled in the art can make any variant or modification in accordance with his spirit.

 In particular, the various modes of sampling the voltage at point P (by conductive stylus or by pressure contact with an underlying conductive sheet) and the various modes of switching the four electrodes in pairs can be combined with one another. This switching can be clocked by a local clock or by a control signal generated by the central unit 36.

 In addition, in a mass production, the individual resistors 16 may take any other form than that described: It may be a resistive strip of uniform section brought into electrical contact at equidistant points with the sheet 10 or else a resistive wire developed in a zig-zag through holes made in the region of the edges of the sheet 10.

 Finally, it should be noted that the invention will, by the high speed at which it is possible to carry out the measurements of xp and yp and by the extremely fine resolution which can be obtained for these values, to carry out recognition of forms, in particular of signatures, with the possibility of dynamic analysis.

Claims (11)

'' CLAIMS 1. A device of the graphic tablet type for converting a plot or the like on a surface into data representative of said plot, for example for the purposes of processing by calculation means, characterized in that it comprises  - a resistive surface (10) of generally rectangular shape, on which said tracing must be carried out using a stylus (30),  - two pairs of rectilinear electrodes (18a, 18b, 20a, 20b) arranged around said surface, the two electrodes of the same pair being essentially parallel and the ends of the electrodes being electrically connected to each other at the four corners of said surface , each electrode being constituted by the series connection of a plurality of individual resistors (16) whose common points are electrically connected to the associated edge of said surface at equidistant points,  - switching and electrical supply means (26, 38; 54) for connecting together, periodically and alternately between the two pairs, the two ends of each electrode of the same pair and for supplying at a given voltage said electrodes; the two pairs of electrodes thus being alternately activated  - means for taking, at each alternation, the potential at the contact point (P) between the stylus (30) and the surface (10), the two pDLentials (Vx, V V) sampled during two alternans  y these successive being representative of the instantaneous coordinates of said point in a reference (xOy) defined by the position of said electrodes. 2. Device according to claim 1, characterized in that the overall electrical resistance of each electrode, in the non-activated state, is substantially lower than the surface resistivity of the surface (10). 3. Device according to one of claims 1 and 2, characterized in that the individual resistances are made from a resistive element of essentially constant section brought at regular intervals into electrical contact with the edges of said resistive surface. 4. Device according to one of claims 1 d 3, characterized in that the resistive surface is of square shape, the four electrodes (18a, 18b, 20a, 20b) being identical. 4. Device according to one of claims 1 to 4, characterized in that the switching and electrical supply means comprise four controlled switches (22a, 22b, 24a, 24b) mounted in parallel with the four electrodes, a unit ( 26) for controlling said switches in pairs, in a clocked fashion, and a DC voltage generator (38) connected to the electrodes at two diagonally opposite corners of said resistive surface (10). 6. Device according to one of claims 1 to 4, characterized in that the switching and electrical supply means comprise four diodes (50a, 50b, 52a, 52b) mounted in parallel with the four electrodes and a generator (54 ) alternating positive and negative square voltages, the ends of an electrode being connected together by the passage of the diode associated with the conductive state during a polarity reversal of the voltage. 7. Device according to one of claims 1 to 6, characterized in that it further comprises an analog / digital converter (34; 34a, 34b) receiving said sampled potentials and the output of which is connected to a central unit (36 ) a computer or the like. 8. Device according to one of claims 1 to 7, characterized in that the sampling means comprise a conductive tip (30a) of the stylus (30) and a flexible metal wire (32) connected to the stylus. 9. Device according to 1- 'one of claims 1 b 7, characterized in that the sampling means comprise a conductive sheet (42) extending parallel to the resistive surface (10), means (44) for maintaining the conductive surface and the resistive surface normally spaced from each other, the pressure exerted by the stylet (30) on the resistive surface causing by deformation a localized contact, at the point considered (P) of the trace, between the resistive sheet and the conductive sheet, the latter being thus brought to the potential (Vx, Vy) of said point (P). 10. Device according to claim 9, characterized in that an ordinary sheet of renewable paper (46) is interposed between the resistive sheet (10) and the stylus (30), the latter being an ordinary writing instrument. 11. Device according to one of claims 1 to 10, characterized in that the switching means operate at a frequency of the order of 100 to 1000 vibrations / second.