Method and apparatus for determining the coordinates of a point on a surface

A screen is disclosed on the surface of which a certain number of resistant conducting emitter strips and insulated conducting receiver strips are laid, the emitter strips being parallel-mounted with the terminals of a voltage generator and the receiver strips being interlaid between the receiver strips such that a conducting receiver strip lies between two resistant emitter strips. The electronics of control comprises an exploring stage to control the receiver strip time-related exploration. Thus, when a sufficiently conducting object used to indicate a point, has touched the surface and bridges an emitter strip and a receiver strip, a higher potential is detected on the corresponding receiver strip and the ordinate Y is given by the strip order number--while the abscissa X is calculated from measuring the potential between each ends of the emitter strip and the object.

The present invention relates to a method for determining the corrdinates 
of a point on surface by pin-pointing it through contact of an object with 
that surface. 
The invention further relates to a device permitting, in particular, 
implementation of this method. 
The known methods of this type generally rely on dividing the surface 
beforehand into a certain number of zones materialized as otherwise and 
selectively sensitive to contact with the object. 
U.S. Pat. No. 3,806,912 presents thus a graphical display board equipped 
with a printed circuit consisting of evenly spaced parallel paths, one 
resistive path lying between two conductive paths. To determine the 
coordinates X, Y of a point, use is made of a stylus comprising one end 
consisting of a conducting material which must establish a negligible 
resistance bridge between three consecutive paths: one resistive path and 
two conductive paths. In this device, the resistive paths are all 
interconnected down to ground via a conductive cross path, whereas one 
path in each conducting pair is interconnected down to ground via one of 
its ends by means of a series resistance cross path, the other conductive 
paths being connected directly by one end thereof to a detection 
apparatus. This device has the drawback of requiring the lay-up of 
horizontal and vertical conductive and resistive paths on the board with 
overlaps for making certain connections. Moreover, coordinate 
determination of a point is carried out by electrically connecting three 
consecutive paths using a metal stylus having a small resistance and 
specially designed for this application. 
Certain methods, which tend in fact to form control key boards utilize 
capacitive keys laid out in a chequered fashion over the surface. Each key 
constitutes a capacitor, whose dielectric varies upon touching the object, 
which can for example be an operator's finger. The associated electronics 
are relatively straight forward, by the location accuracy is rather low 
since the keys are fixedly positioned with a relatively large pitch. 
Better accuracy is achieved with those methods employing an electrically 
active object, known as "light pen", which cooperates with a surface in 
the form of a cathode ray tube. The location of the contact point results 
from interaction of the object with the electronic scan of the screen. The 
equipment that these methods imply involves extensive and complex 
electronics stages, that are essentially computer-associated. 
The main object of this invention is to provide a method and a device for 
coordinate determination which conciliates good location accuracy and a 
straightforward electronics infrastructure. 
This problem is resolved, in accordance with the invention by performing 
the following operations: 
(a) a certain number of resistant conducting emitter strips are laid on the 
surface parallel-mounted with the terminals of a voltage generator, as are 
a certain number of insulated conducting receiver strips interlaid between 
the foregoing such that a conducting receiver strip lies between two 
resistant emitter strips; 
(b) the surface is touched with a sufficiently conducting object having 
dimensions such that it establishes an electrical bridge between a 
resistant conducting emitter strip and a conducting receiver strip; 
(c) the insulated strips in which there has been a variation in potential 
is determined whence one of the contact point coordinates is deduced; 
(d) this variation in potential is measured in order to deduce the other 
contact point coordinate. 
The object is not electrically active. It merely needs to be sufficiently 
conductive to make a connection between two neighbouring strips, one 
emitter and one receiver. This object may then be an operator's finger. 
This technique therefore does away with any electronic infrastructure 
associated with the object. 
Furthermore, this obviates any division of the surface into a criss-cross 
of keys set-out in a fixed pattern thus making it possible to obtain 
excellent point position definition by varying the width and the pitch of 
the strips, on a basis compatible with the object-surface contact area 
dimensions. 
In a preferred embodiment of the method, the potential of the receiver 
strips can be cyclically explored until a potential in one of these strips 
different from that in the other strips is detected, whereupon the 
exploration is halted in order to measure this potential. 
By way of this configuration, just one measurement apparatus proves 
necessary. 
In a first embodiment variant of the method, a known potential is fed into 
one end of each emitter strip, the other end of each strip being connected 
to ground. 
Once the potential has been measured, the emitter strip polarity is 
reversed in order to take a further measurement, and this cycle is 
repeated a certain number of times. 
These operations make it possible to eliminate certain unknown constant 
factors, such as the object's resistance, or certain possibly variable 
factors, such as the supply voltage. 
In a second embodiment variant of the method, an alternating potential with 
a frequency of approximately 1 kHz is fed into one end of each emitter 
strip with the other end of each strip connected down to ground and the 
measured voltage is filtered. 
This therefore eliminates any parasite voltages induced by the 
surroundings, notably at the mains frequency of 50 or 60 Khz. 
As before, the potential is successively measured a certain number of 
times, the alternating potential and ground reversed and then the same 
number of measurements taken again. 
In every event, an average value is calculated over the potentials measured 
during the successive readings to minimize any possible fortuitous errors. 
Lastly, the exploration is resumed once all the emitter strips have 
reverted to the same potential. 
In a further aspect of the invention, the device for determining the 
coordinates of a point on a surface comprising a certain number of 
conductive strips (E.sub.1 to E.sub.5, R.sub.1 to R.sub.4) to form 
separate zones selectively sensitive to the contact of an object is 
characterized in that it comprises, on the surface, a certain number of 
resistant conducting emitter strips each having one end terminal for the 
ground connection and one end terminal for the voltage generator 
connection, and a certain number of conducting receiver strips interlaid 
between the emitter strips such that a conducting receiver strip lies 
between two resistant emitter strips where each of these conducting 
receiver strips has an end terminal for a connection in parallel with a 
potential measuring apparatus and offers a virtually constant linear 
resistance over its whole length.

With reference to FIG. 1, the device comprises a transparent surface 1, 
made of sodio-calcic glass for instance, which forms a cathode display 
screen. Arranged on this surface are a certain number of parallel emitter 
strips E.sub.1 to E.sub.5, e.g. in thin oxide SnO.sub.2 doped with 
fluorine deposited by pyrolysis. These strips are conductive and have a 
thickness such that the resistance of strip section equal in lenght to its 
width is in the order of 100 to 200.OMEGA.. 
The strip ends 10 are neutralized by an edging 2 of the surface and serve 
as electrical connections with the collecting strips 3, 4. These strips 
can be made up of a metal deposit that is conductive and/or can be 
soldered in silver or nickel deposited for example in the form of a 
conductive paint, resin or enamel by serigraphy. 
Receiver strips R.sub.1 to R.sub.4 are interlaid between the emitter strips 
E.sub.1 to E.sub.5 end are formed in appreciably the same fashion and each 
comprise one neutralized end constituting the connection terminal 11. 
To clarily matters, the strip neutralized parts and the collecting strips 
have been shaded in. 
In the example described, each strip is 3.5 mm wide and the distance from 
one strip to the next is also 3.5 mm. The presence of these strips has 
practically no effect on the visibility through the transparent surface 1. 
The collecting strips 3, 4 are wired to a switch 5 which either connects 
the strip 3 to a voltage generator 6 and the strip 4 to ground M (FIG. 1), 
or, inversely, connects the strip 3 to ground and the strip 4 to the 
generator 6. This switch is activated by a control mechanism 7 which will 
be covered in more detail later. 
The receiver strips R.sub.1 to R.sub.4 are wired to an exploring stage 8 
which successively connects those strips to a voltmeter V. 
Utilization of the apparatus whose principle has just been described 
consists of determining the coordinates X and Y of a point on the surface 
that is materialized by the contact of a conducting object 9 (FIG. 1) such 
as an operator's finger 9a (FIG. 3). The object's dimensions must be such 
that it covers two adjacent strips e.g. E.sub.2 and R.sub.2 (FIG. 1), 
setting up an electrical bridge P between said two strips. 
In the example shown in FIG. 1, the receiver strip R.sub.2, by contact with 
the object, is set at a certain potential detected by the voltmeter V, and 
it is supposed that the ordinate Y is given by the strip order number. 
The abscissa X is calculated from the sketch in FIG. 2, on which it has 
been assumed that the voltage generator is a generator 6a delivering a 
steady voltage U, of around 15 volts. 
If L is the useful length of a receiver strip, then as a result of the high 
internal resistance in the voltmeter V: 
##EQU1## 
where u is the voltage measured by the voltmeter 
If the operation is repeated by inverting the generator 6a and the ground 
M, then a new voltage u' is measured, giving thus: 
##EQU2## 
whence: 
##EQU3## 
It will be observed that this method renders the measurement independent of 
the supply voltage U accidental variations in which might adversely affect 
the measurement, and independent of the resistance value of the contact 
bridge established by the conducting object 9 providing this resistance is 
decidely lower than the voltmeter internal resistance to avoid disturbing 
the measurement. In one embodiment example, a voltmeter with an internal 
resistance of 2 M.OMEGA. has been chosen in the knowledge of the fact the 
resistance of the operator's finger measured at an applied voltage of 
frequency 1 kHz is around 500 k.OMEGA.. It can thus be seen that 
conducting object 9 over a very wide resistance range may be used, from 
very low values upto values in the order of 1 M.OMEGA. providing a 
voltmeter with a suitable internal impedance is chosen, i.e. with a much 
higher impedance. 
A description will now be given, with reference to FIG. 4, of the 
electronics surrounding the device and the method description completed. 
In this drawing, the strips have been shown in greater numbers than in FIG. 
1, but this number is essentially variable in terms of the surface 
dimensions. Those elements already described have been reshown with the 
same reference numbers. These elements are for the most part the supply 
inverting switch 5, the generator 6, the exploring stage 8 and the 
voltmeter V which is of the digital type. 
A control stage 12 of a calculating unit 13 is wired to the exploring stage 
8 to control the receiver strip time-related exploration. In the example 
described, the timing gives a 5 ms pause before going onto the next strip. 
The voltmeter V is wired to an input stage 14 on the unit 13 for 
transmitting a message TAD to said stage where said message comprises the 
digital value of the measured voltage and the address of the receiver 
strip where this measurement was taken. 
A further connection permits transmission of a signal FC representing the 
end of analog-to-digital conversion. 
The control stage 12 is also wired to the voltmeter V for sending it a 
conversion control signal CC. 
Lastly, the exploring stage 8 is controlled by the stage 12 which sends it 
an exploration timing signal EX, and the switch 5 is, via the line 7, 
controlled by the same stage 12 which sends it a signal INV. 
The calculating unit 13 further comprises an encoding stage 15 wired to a 
display unit 16 indicating the measured coordinates X and Y. 
A description will now be given, notably in reference to FIG. 5, of how the 
overall device works and of the measurement method. On the diagram in FIG. 
5, the measurement sequence runs from the top to the bottom. 
Initially, the exploring stage 8 successively sweeps the receiver strips at 
a rate of 5 ms per strip and in the absence of the object 9, measures a 
zero potential each time or, to be more exact, a potential below a 
predetermined threshold S. 
Once the object has touched the surface 1, a potential higher than the 
threshold S is detected, causing thus the emission of a signal D toward 
the calculating unit 13. The receiver strip where the detection occurred 
is stored as a number to give the ordinate Y. The control stage 12 then 
sends the order INV to invert the polarity across the emitter strips, 
followed by a voltage measurement and digital conversion order CC. When 
the conversion has been completed, the stage 8 emits an order FC which 
causes storage ST of the converted measurement u. 
By means of a new order INV, the polarity on the emitter strips is reversed 
and a new measurement u' is taken and stored. 
This process is repeated five times. The first measurement against each 
polarity is then eliminated, to avoid any transient phenomena. Lastly, the 
averages of the u and u' measurements are calculated to deduce the 
abscissa X by means of the formula given earlier. 
The results are finally indicated on the display device 16. 
The calculation process is halted and the stage 8 resumes sweeping only 
once the detected receiver strip potential has dropped back below the 
threshold S, namely when the object 9 is no longer in contact with the 
surface. 
All the foregoing applies to the case of the generator 6 delivering a 
steady voltage. 
As a variant, an alternating voltage generator may be used with a frequency 
of 1 kHz. Under these circumtances, an active band-pass filter 17 
corresponding to the 1 kHz band is positioned between the exploring stage 
8 and the voltmeter V followed by an active rectifier 18 which delivers a 
steady voltage to the voltmeter input. 
The measuring method is virtually the same, save that five successive 
measurements are taken with the generator on one side and the ground on 
the other, and then five more after having inverted the generator and the 
ground. 
The advantage of using alternating current is that, as a result of the 
filtering, the 50 or 60 Hz components induced by the surroudings are 
eliminated, as are any possible interference effects brought about by 
thyristors in the vicinity. 
In all events, the invention by indicating with the finger makes it 
possible to define the coordinates of any point on a fixed or moving image 
projected onto the surface 1, and to repeat this operation at close 
intervals in time. The accuracy achieved is excellent and depends on the 
surface area of the contact object used, e.g. the finger. The apparatus is 
also most reliable since if some dirt were to fall on the surface, then 
the short-circuit it would cause would appear as coordinates in the 
absence of the object and thus reveal it immediately. 
The invention further provides for the predetermining of the abscissa 
coordinates of contact zones by arranging the widened parts on the strips 
(FIG. 6), for the purposes of a conversational use upon display. By way of 
an example, widened sections 21 can be positioned on the emitter strips E 
for defining keys corresponding to alphanumeric values. 
In another configuration, the emitter strip can be split into E' and E" on 
either side of the receiver strip R' and comprise widened sections 22' and 
22". 
Of course, the invention is not restricted to the examples described but 
covers numerous variants accessible to specialists in the field. The 
strips could take forms varied in terms of the application such as, for 
instance, a greater density there on the screen whose finer resolution is 
needed, or instead of being rectilinear have a sinusoidal or castellated 
form, or be circular and concentric for certain and radial for the others 
and thereby define a point in polar coordinates. 
Likewise, the strips could consist of deposits other than tin oxide, for 
example, indium or cadmium oxide or other mixtures of various metal 
oxides, be doped with dopants other than fluorine, for example antimony 
and be deposited by vapour phase or cathode projection. 
They could, moreover, be deposited with no difficulty in thicker layers 
that are more resistant to abrasion than in the afore-described example 
offering thus a very wide surface resistance range dropping for instance 
down to 10.OMEGA. for a strip section equal in length to its width. 
These strips could have different widths and spacing than those mentioned, 
possible only providing they can be easily connected electrically via the 
finger or any other conducting instrument such as, for example, the tip of 
a conducting rubber rod or a stylus with flexible metal conducting strands 
having dimensions suitably matched with the width and spacing of the 
emitter and receiver strips. 
Furthermore, for the purpose of easing the keyboard-to-electronic circuitry 
connections, all the receiver R.sub.1 to R.sub.4 and emitter E.sub.1 to 
E.sub.5 strip connections could be routed to one and the same side of the 
screen. 
Additionally, the strips 3 and 4 could be done away with, in which case the 
necessary connections would be ensured by a strip of rubber, of the Zebra 
type for example, composed in a known fashion with alternate conducting 
and resistant bands whose pitch would be chosen to match the connection 
with the network of emitter (E.sub.1 to E.sub.5) and receiver (R.sub.1 to 
R.sub.4) strips on the one hand and on the other hand with a flexible 
printed circuit linking through to the detection system. This rubber 
element would be secured in a suitable manner between the screen plate and 
the flexible printed circuit. 
Moreover, the strip support can be composed, as described in the example, 
directly by the front face of a cathode ray tube and also the front face 
of any other type of display tube such as plasma, LED or liquid crystal 
display systems; the support can also constitute a screen that is placed 
in front of another screen like those mentioned above or in front of a map 
(street, or town map etc.). 
In the latter cases, this support can be made of glass but also any other 
insulating transparent mineral or organic substance, a hard plastic such 
as Perspex or soft plastic such as sheet polycarbonate providing the 
surfaces are treated in such a way that conducting strips having the same 
transparency, hard-wearing and electrical resistance characteristics can 
be laid in place.