WO2011098854A9

WO2011098854A9 - Method for the detection of a body with respect to a surface, detecting device for the implementation of the method, and surface comprising such devise

Info

Publication number: WO2011098854A9
Application number: PCT/IB2010/000690
Authority: WO
Inventors: Marc Gresset
Original assignee: Varidal Company Limited
Priority date: 2010-02-11
Filing date: 2010-02-11
Publication date: 2012-03-22
Also published as: WO2011098854A1

Abstract

Method for the detection of a body (5) with respect of a surface (1), said surface (1) comprising a capacitive sensor (2) whose capacitance (C) is variable, said method comprising the following steps: measuring the sensor (2) voltage; comparing the sensor (2) voltage with a reference voltage; generating a first signal (S1) representing the sensor (2) voltage, said signal (S1) taking two states: a high state, wherein the sensor (2) voltage is equal to or greater than the reference voltage, a low state, wherein the sensor (2) voltage is smaller than the reference voltage; if the first signal (S1) is in the high state: incrementing a variable with respect to a clock (17) as long as the first signal (S1) is in the high state; comparing the variable with a threshold value; when the variable reaches the threshold value, emitting an output signal (S2); if the first signal (S1) is in the low state, initializing said variable.

Description

METHOD FOR THE DETECTION OF A BODY WITH RESPECT TO A SURFACE, DETECTING DEVICE FOR THE IMPLEMENTATION OF THE METHOD, AND SURFACE COMPRISING SUCH DEVISE

FI ELD OF THE I NVENTION

The invention relates to the detection of a body with respect to a surface. More precisely, the invention relates to the detection of a person on a surface for safety reasons, by using capacitive sensors.

BACKGROUND OF THE INVENTI ON

Sensors are instruments transforming the measure of a physical quantity into a signal readable by an operator.

The physical quantity can be of various kinds: quantity of light, pressure, sound, temperature, current, or distance.

Capacitive sensors are often used for the measure of distance. Those sensors have a variable capacitance term , depending on the presence and the distance of a body to the sensors.

The capacitance between two electrodes can be commonly calculated by the following formula:

c = ^

d

where C is the capacitance, ε is the dielectric permittivity of the medium between the two electrodes, S is the effective electrode surface and d is the distance between the electrodes.

Capacitive sensors operate by using capacitance variations. I ndeed , from the previous formula, it can be deduced that a variation of the dielectric permittivity or of the distance between electrodes, or a combination of the two variations, induces a variation of the capacitance.

Such variations can be produced by the presence of an object - the object to be detected.

Such sensors can be found for instance in touch screen technology.

Document US 2008/01 1714 (Kremin) presents an example of such screen. The sensor is set by two coplanar electrodes, standing as an open capacity, whose capacitance varies when an object - a finger or a stylus for instance - is approached. Sensors can be placed in rows or columns. A multiplexer rules the sequential measure of each sensor.

I n order to determine the location of the body to be detected, the document US 4,686, 332 (I BM) proposes a system using a pattern of rows and columns of wires, associated by pair, each pair constituting a capacity. When an object is approached near a pair, the capacitance varies. First multiplexer measures alternatively each row, and second multiplexer is allocated to the columns. When the system receives a signal that an object is near the screen, the multiplexers are activated to determine which pair of wires has the highest capacitance, among each row and column. Thus, coordinates on the screen can be calculated.

Such sensors are complicated to implement for touch screens. Indeed, they required a response time as small as possible for the user comfort. Moreover, high precision must be obtained so that the localization of the object on the screen can be aimed toward an application. Rapidity and precision considerations greatly increase the cost of components and the complexity of touch screens.

Great sensitiveness is also required, so that the user do not have to exert an important pressure on the screen and break it.

Another interesting application of capacitive sensors is floor detection systems.

Document US 6,515, 586 (WYMORE) describes an example of such system , which comprises a floor formed by a layer of sensors. The sensors are arranged in line, each line being connected to a multiplexer. A controller read information from the multiplexer and generates a signal with respect to a program.

Document JP 10-21 3499 also proposes to use a pattern for floor detection.

Another example is done in document US 5,798,703 (SAKAI) for floor detection, using an alternate technology. A layer is provided with electrodes pairs, each electrode comprising two plates generating a short circuit when in contact. When one of the pairs or both are in short-circuit, the output does not receive signal anymore, so that a system can determine that somebody is on the layer. This system can be used for example for detecting the presence of an operator in a restricted zone, and shutting any dangerous machine.

Previous devices propose solutions which are incomplete. Indeed, they impose that any contact generates a signal or an alarm, so that their applications are restricted: they can be used only as exceptional implementation , and do not suffer any adaptation . SUMMARY OF THE I NVENTION

For these purposes, the invention provides, according to a first aspect, a method for the detection of a body with respect of a surface, said surface comprising a capacitive sensor whose capacitance is variable, said method comprising the following steps:

- measuring the sensor voltage;

- comparing the sensor voltage with a reference voltage;

- generating a first signal representing the sensor voltage, said signal taking two states:

- a high state, wherein the sensor voltage is equal to or greater than the reference voltage,

- a . low state, wherein the sensor voltage is smaller than the reference voltage;

- if the first signal is in the high state:

- incrementing a variable with respect to a clock as long as the first signal is in the high state;

- comparing the variable with a threshold value;

- when the variable reaches the threshold value, emitting an output signal;

- if the first signal is in the low state, initializing said variable.

By implementing this method, one can quickly and reliably generates a signal indicating the presence of a body, without working at high frequencies, hazardous for human life.

The method can incorporate timer steps, consisting in, when the first signal is in the hig h state and the variable value reaches the threshold value:

- defining a new threshold value superior to the previous threshold value;

- comparing the variable to the new threshold value;

- when the variable value reaches the new threshold value, emitting an output signal;

These steps are preferably repeated a determined number of time.

The method can then overcome interferences problems, for instance occurring when the sensor is disturbed by its environment.

The invention provides, according to a second aspect, a detecting device for the detection of a body with respect of a surface, said device comprising:

- a capacitive sensor whose capacitance is variable;

an oscillating circuit measuring the capacitive sensor voltage;

- a control system coupled to a clock, for the implementation of said method;

means for generating an output signal.

The detecting device can then easily be implemented in various systems, in various arrangements, in order to generate a signal indicating the presence of a body.

The detecting device can comprise a frequency divider at the entry of the control system , in order to implement timer steps. The invention provides, according to a third aspect, a sensing surface for the detection of a body, comprising :

an array of said detecting devices;

- a binary memory recording which detecting devices generate an output signal;

By reading the binary memory, one is able to determine which detecting devices generate a signal indicating the presence of a body with respect to the surface, so that the body can be localized upon the surface.

Advantageously, the binary memory forms binary words, the output signal setting to one a bit of a word, providing an easy and efficient method of reading the binary memory: the value of the binary words indicates if there is a presence of the body, for how long this presence has been detected, and which detecting devices generate the signal.

The array comprises for instance rows of detecting devices superimposed to columns of detecting devices, each detecting device being independent from the other. The array forms a mesh providing reference for the localization of the body with respect to the surface.

I n a preferred embodiment, a binary word is formed from columns and a binary word is formed from rows, the comparison of the two words giving the crossing points of sensors generating an output signal for the localization of the body.

The sensors are preferentially laid upon an electrically insulated material , to avoid interferences with the material under the surface. The sensors can then be integrated in a concrete screed, in order to provide a surface which can be used as a floor or a wall for instance.

BRI EF DESCRI PTI ON OF THE DRAWI NGS

FIG. 1 and FIG. 2 are schematic views of a capacitive sensor;

FIG. 3 is a planar view of a detecting surface comprising an example of array of capacitive sensors;

FIG. 4 is a cross section of the detecting surface of FI G .3 in a first embodiment;

FIG. 5 is a cross section of the detecting surface of FIG.3 in a second embodiment;

FIG. 6 is a schematic electrical representation of a capacitive sensor;

FIG. 7 is a diagram representing the signal variations of a capacitive sensor in a first state;

FIG. 8 is a diagram representing the signal variations of a capacitive sensor in a second state;

FIG. 9a and FIG. 9b are diagrams representing the signal variations of a capacitive sensor respectively in two states when the sensor is in the air;

FIG. 10a and FIG. 10b are diagrams representing the sig nal variations of a capacitive sensor respectively in two states when the sensor is in contact with a floor;

FIG. 1 1 is block diagram illustrating the generation of a signal from one sensor;

FIG. 12 is a block diagram illustrating the generation of a signal from three sensors;

FIG. 13 is a block diagram illustrating the steps of the generation of a signal;

FIG. 14 is a planar view of a detecting surface wherein a signal is generated from three sensors, when bodies are placed near the surface;

FIG. 15 is a planar view of the detecting surface of FIG. 3 when a body is placed near the surface;

DESCRI PTION OF PREFERRED EMBODIMENTS

I n FI G. 3, 14 and 1 5, it is represented a sensing surface 1 , for the detection of a body, based upon the contact of the body on the surface 1 , the localization of the body on the surface, and the time during which the contact is established. The surface 1 can be integrated for instance on the floor or on the wall in a room.

The sensing surface 1 uses capacitive sensors 2. An example of the operation of sensors is made by reference to FIG. 1 and 2. A first electrode 3 of the capacitive sensor is for instance formed by an electrical wire, the ground forming the second electrode 4. The capacitance between the two electrodes can be calculated from the formula given in introduction and is called the reference capacitance C_ref (FIG . 1 ). When a body is approached to the electrical wire 3, it stands as a third electrode 5, so that a second capacitance, called the variable capacitance C_var, can be calculated between the wire 3 and the body 5 (FIG. 2). The resulting capacitance C is the sum of the two capacitances C_ref+C_var. I n another embodiment, the capacitive sensor 2 comprises one unique electrode, the capacitance being formed only when the body to be detected is approached . Thus, a measure of the capacitance gives an indication on the presence of a body to be detected .

In a particular implementation, the sensor 2 is made from a bundle of electrical lines 6. Lines 6 are advantageously placed in a coplanar arrangement, and are electrically insulated from each other. I n a preferred embodiment, sensor 2 comprises a bundle of ten electrical lines 6, wherein nine lines are standing as a first electrode 3, and the tenth line is connected to the ground, standing as the second electrode 4.

No requirement of electrical conduction value is demanded for the body 5 to be detected, as long as it is not a perfect insulator, so that an electrical current can circulate between the first and third electrodes 3, 5.

The sensing surface 1 comprises detecting devices. A detecting device converts the measured capacitance variations of a sensor 2 to a signal by measuring the voltage of an oscillating circuit 7.

The capacitive sensor 2, modeled as a capacitor 8, is linked to a resistor R in a voltage divider RC circuit, as illustrated on FI G. 6. The resistor R can be a rheostat in order to allow the adjustment of the circuit 7 characteristics.

The circuit 7 is looped with two Schmitt triggers 9 as follow. The potential of the capacitor 8 is sent to the input of two Schmitt 9 triggers. The outputs 10, 11 of the two triggers 9 form the input of a logic gate 12 NAND, the output 13 of the gate 12 being used as the supply potential V₂ of the RC circuit.

The sensor 2, the resistor R and the Schmitt triggers 9 form an oscillating circuit 7, whose equation can be expressed as follow: where T=RC is the time constant of the circuit 7.

The solution V-i of the equation is oscillating, following charge and discharge cycles of the capacitive sensor 2, 8.

The output potential V₂ of the gate 12 is a well-strobe signal giving an inverted image of the potential Vi of the sensor 2: when the potential reaches a reference voltage - the triggers 9 reference voltage -, the output potential of the triggers 9 changes between two values. A second group 14 of two triggers 15 with a NAND gate 16 can be used to reverse the signal.

Consequently, the resulting signal S1 corresponding to the output of the oscillating circuit 7, is a well-strobe signal, indicating when the capacitor 2, 8 voltage has reached a determined value, and stands as a measure of the capacitive sensor voltage V-|.

The resulting signal S1 can be in only two states:

-a high state, wherein the capacitor 8 voltage is equal to or beyond the determined value;

-a low state, wherein the capacitor 8 voltage is under the determined value.

In order to obtain measurable variations, the working frequency of the oscillating circuit 7 is chosen near the cut off frequency f_c of the RC circuit. The cut off frequency f_c is determined as follow:

1 1

2πτ 27iRC Besides, the resistance R and the capacitance C are set so that the working frequency is below 50 kHz, and preferentially around 1 8 kHz, in order not to interfere with human resonance frequencies, and to limit health risks.

This choice of such low frequency can bring a problem . Indeed, the frequency variations due to capacitance variations are too small to ascertain capacitance variations. Thus, the detecting device does not count the cycles of the signal S 1 in order to detect capacitance variations, but it is based on the time.

The time constant τ is an indicator of the velocity of the capacitor 8 cycles. The greater the time constant τ is, the quicker the capacitor 8 voltage reaches the determined value and the longer the resulting signal S 1 is in the high state.

Thus, when the capacitance C of the sensor 2 is modified by a body, the capacitor 8 voltage V1 cycles are modified , as is the resulting signal S 1 . Besides, the resulting signal S1 presents bands corresponding to the high state, the width of those bands being variable with the capacitance of the sensor 2.

The detecting device includes a control system 17 , coupled to a clock 1 8. The resulting signal S 1 is sent to the control system in order to be analyzed.

I n an embodiment, the resulting signal S 1 is amplified before being sent to the control system 17. For instance an amplifier 19 such as frequency divider is placed at the entry of the control system 1 7, the band width of the resulting signal S 1 being increased.

Based upon the clock 18 frequency, the control system 1 7 increments a variable A as long as the resulting signal S 1 is in the high state. For that purpose, the control system 1 7 establishes when the resulting signal S 1 is equal to a maximum value, and when not.

When the resulting signal S 1 decreases to the low state, the variable A is re-initialized.

Consequently, by implementing a threshold value A₀ corresponding to a period of time with respect to the clock 18 frequency and how the variable A is incremented, the detecting device is able to define a time limit for the resulting signal S 1 to be in the high state. Indeed, by comparing the variable A with the threshold value A₀ at each clock 1 8 turn, the system 1 7 determines two states for the sensor 2:

-as long as the variable A is under the threshold value A₀, the sensor 2 is in a low state, no body acting upon its capacitance;

-when the variable A reaches and goes beyond the threshold A₀, the sensor 2 is activated , a body being detected .

The incrementation of the variable A can be linear. I n a preferred embodiment, the incrementation of the variable A is cubic, so that the threshold value A₀ can be set as a high value, increasing precision.

Then , the control system 1 7 is able to generate a second signal S2 indicating that the sensor 2 is activated.

As the surface 1 can be integrated in the floor of a room , interference phenomena can appear. In particular, it has been noticed that when the surface 1 is integrated in concrete floor, as it will most commonly be, the variations of the band width of the resulting signal S 1 are too unsteady to be detected.

FIG . 9 and 1 0 illustrate the phenomena. The scale is not accurate, these figures standing as simple principle illustrations. When the detecting surface is in the air (FI G . 9a) , the band width of the resulting signal S 1 varies in a consistent way, so that even if there are interference phenomena (hatched area), they are greatly absorbed by the variations induced by the presence of a body (FIG. 9b) .

On the contrary, when the detecting surface is in contact with concrete, the band width variations are absorbed by the interference (hatched area), so that the overcome of the threshold value A₀ by the variable A is difficult to ascertain: when the variable A reaches and overcomes the threshold value A₀, it can be the result of the interferences or of the capacitance variation (FIG . 10a and 1 0b).

I n order to solve this problem , the control system 1 7 determine the time during which the variable A is equal to or beyond the threshold value A₀ to validate the emission of the second signal S₂. For instance, the 5 control system 17 comprises a timer, which increases the threshold value A₀ when the variable A reaches or is beyond the threshold value A₀. The new threshold value A'₀ is then compared to the variable A in order to determine if the variable A is always beyond the threshold. The new threshold value A'₀ can be again increased and compared to the variable 10 A. These operations can be repeated as many times as wished, in order to determine the threshold A₀ for which the variable A is no greater, giving information about the weight of the body. However, the more they are repeated , the longer the control system 17 will be to emit the second signal S2.

15 The detecting device can then generate an output signal S2 when a body 5 is on the detecting surface 1 for a precise period of time.

As capacitance variations are detecting by determining the period of time during which the resulting signal S 1 is in the high state, the detecting device allows greater precision than devices based upon the detection of

20 frequency variations.

Moreover, reactivity of the device is increased. Indeed, frequency variations are relatively long to detect, as they suppose analyzing a sample of the signal comprising several cycles, instead of counting the length of the high state on one cycle as described here above.

25 Thus, the frequency of the cycles needs not to be increased, allowing the detecting device to work at low frequency, around 18 kHz as mentioned here above.

In order to localize the body 5 on the detecting surface 1 , the detecting surface 1 comprises an array 20 of detecting devices. The array 30 20 can be of any type, 2D type or 3D type. Each detecting device can generate an output signal S2, which is translated in a binary memory 21 as a setting to one. By regrouping adjacent detecting devices, for instance eight adjacent detecting devices, a binary word 22 - such as an octet - can be formed: when a body 5 is detected on the surface 1 by some detecting devices, the bits allocated to those detecting devices in the binary memory 21 are set to one.

A word 22 contains three pieces of information:

- the presence of a body 5 on the surface 1 ;

- which sensors 2 are activated;

- the sensors 2 have been activated for a determined period of time.

I n a first embodiment (FIG. 14) , the sensors 2 are arranged in parallel rows 23. Each sensor 2 operates as an independent capacitor. Sensors 2 can be directly laid on the floor and covered by any flooring . Alternatively, sensors 2 can be laid upon an electrically insulated layer 24, in order to be insulated from the ground. In order to maintain the sensors 2 precisely localized upon the insulated layer 24, sensors 2 can be glued 25. A detecting device can be allocated to one or more sensors 2. For instance, three sensors 2 can be part of one detecting device. In that case, the oscillating circuit can be modeled as comprising three capacitors in parallel (FIG. 1 2).

The presence of a body 5 upon any of the sensors 2 triggers a capacitance variation, and this variation is proportional to the number of sensors 2 of the detecting device that detect the body 5. Consequently, for instance by setting the threshold A₀, the resulting signal S2 can be generated only if a determined number of sensors 2 of a same detecting device simultaneously detect the body 5, so that the capacitance variation is consistent enough to be detected.

FIG . 14 illustrates that first embodiment: although a body 5A detected by only one sensor 2 from a bundle of three sensors 2 will not set to one the bit allocated to the bundle, a bigger object detected for instance by two sensors 2 of a group of three will trigger a resulting signal S2. Only bodies 5B presenting a dimension bigger than the distance between the sensors 2 will generate a resulting signal S2, so that the detection can be directed toward a class of bodies.

For instance, a human body will be detected, whereas a cat will not be seen . Moreover, a person standing on the floor will not generate a resulting signal S2 if the sensors 2 are set so that the feet do not cover simultaneously two sensors 2. Only if the person is lying on the floor, covering two or more sensors 2, will a resulting signal S2 be generated.

The value of a binary word 22 then indicates the presence of a body 5 corresponding to a determined class with respect to the surface.

In a second embodiment, the sensors 2 are arranged in parallel rows 23 crossing parallel columns 26 (FIG . 3 and 1 5). The sensors 2, and the detecting devices, are as much as possible independent from each other. Indeed, the electrical lines 6 standing as sensors 2 are electriclally insulated from each other. For instance, the sensors are embedded in an insulated material 27, such as epoxy. As previously, the sensors 2 can be preliminarily laid upon an insulated layer 24.

Advantageously, one unique detecting device is associated to each sensor 2. By comparing the words 22 formed from the columns 26 with the words 22 formed from the rows 23, a processing system can then define the crossing points 28 on the surface 1 of the activated sensors 2. Those crossing points 28 match with a zone surrounding the body 5 to be detected. The processing system can also estimate its size by identifying the number of adjacent crossing points 28.

The detecting surface 1 can be used as a person detection system .

The sensors 2 define a mesh whose dimensions are adapted for the detection of a human body. For instance, the sensors 2 define square elements of about 30 cm of dimension. In any case, the dimensions are chosen in accordance with the size of the body to be detected, so that at least a row 23 and a column 26 are activated . The processing system can be configured in order to determine if a problem has occurred, such as the fall of a person lying on the floor.

I ndeed , based upon the value of the binary words 22, the processing device can determine the nature of the contact:

- if a person is standing , the number of crossing points 28 of activated sensors 2 approximately corresponds to the feet surface;

- if a person is walking , the time of the contact is relatively short;

- if a person is lying on the floor, the number of crossing points 28 is increased, as is the time of the contact.

Consequently, by implementing in the processing system a reference value for the binary words 22, a difference between a person walking or standing on the surface 1 , which means that everything is all right, and a person lying on the surface 1 , which can mean that a problem is occurring , can be made.

The array 20 of detecting devices can be directly laid on the floor of a dwelling place, for instance in a flat or a house. Connections can be integrated in walls. The detecting surface 1 can extend over the whole place, or only over some rooms where a watch is required.

Alternatively, the array 20 of detecting devices can be integrated in a concrete screed 29. Advantageously, the electrical lines 6 standing for the capacitive sensors 2 are coated in an electrically insulated material 27 so that they are insulated from each other. A first electrically insulated layer 24 is applied on rough floor in order to insulate the electrical lines 6 form the ground; the sensors 2 are laid upon the first layer 24, and are then embedded in a concrete layer 29, superimposed to the first layer 24. A finishing layer 30 can be applied upon the concrete layer 29. On the one hand, this configuration provides an effective insulation from the earth and on the other hand, this configuration maintains the performances of the detecting devices.

The detecting surface 1 can be configured so that the localization of bodies which do not move, such as the furniture of the dwelling , are recorded as constant contact. This can be translated in an initial value of the binary words 22 different from 0.

When a problem is detecting , the processing device can generate an alarm signal, which is sent for instance to a remote watching interface. An operator is then informed that a body 5 is on the surface 1 for a determined delay and can estimate its localization on the surface. Actions can be taken in consequence, for instance phoning to the dwelling place or sending assistance. The alarm signal can be sent by any means. Advantageously, the alarm signal is sent by wireless means.

The detecting surface 1 will find a particular application for elder people living alone. The detecting surface 1 will allow those people to stay independent at home and being comforted by a non intrusive watching .

In another embodiment, the detecting surface 1 is used as an alarm for intruders: the surface 1 is laid on the floor, near access points, and as soon as a contact is detected, that is to say when the output signal S2 is generated, an alarm is raised.

The detecting surface 1 can also be used as a tracking surface: by reading the binary words 22 at regular interval, a body 5 on the surface can be tracked .

The detecting surface 1 can be integrated in any surface, for instance in floor, wall or in furniture such as table.

The detecting surface 1 described herein is simple to implement. It can be delivered as a floor - or any other surface - covering. A finishing layer 30 chosen among many different materials, such as wood, carpet or tiles, can be used , so that the surface is aesthetically enhanced and it can be integrated in a dwelling place.

The detecting device assures a quick and reliable detection of a body 5, based upon capacitance variations.

Claims

1. Method for the detection of a body (5) with respect of a surface (1), said surface (1) comprising a capacitive sensor (2) whose capacitance (C) is variable, said method comprising the following steps:

- measuring the sensor (2) voltage;

comparing the sensor (2) voltage with a reference voltage;

generating a first signal (S1) representing the sensor (2) voltage, said signal (S1) taking two states:

- a high state, wherein the sensor (2) voltage is equal to or greater than the reference voltage,

- a low state, wherein the sensor (2) voltage is smaller than the reference voltage;

- if the first signal (S1) is in the high state:

- incrementing a variable (A) with respect to a clock (17) as long as the first signal (S1) is in the high state;

- comparing the variable (A) with a threshold value (A₀);

- when the variable (A) reaches the threshold value (A₀), emitting an output signal (S2);

- if the first signal (S1) is in the low state, initializing said variable (A).

2. Method according to claim 1, wherein when the first signal (S1) is in the high state and the variable value (A) reaches the threshold value (Ao), said method comprises the following steps:

- defining a new threshold value (A'₀) superior to the previous threshold value (A₀);

- comparing the variable (A) to the new threshold value (A'₀);

- when the variable value reaches the new threshold value (A'₀), emitting an output signal (S2);

3. Method of claim 1, wherein the steps according to claim 2 are repeated for a determined number of times.

4. Detecting device for the detection of a body (5) with respect of a surface (1), said device comprising:

a capacitive sensor (2) whose capacitance is variable;

an oscillating circuit (6) measuring the capacitive sensor (2) voltage; - a control system (17) coupled to a clock (18), for the implementation of the method according to 1 to 3;

means for generating an output signal (S2).

5. Detecting device according to claim 4, comprising a frequency divider (19) at the entry of the control (17) system.

6. Sensing surface (1) for the detection of a body (5), comprising: - an array (20) of detecting devices according to claim 4 or 5;

a binary memory (23) recording which detecting devices generate an output signal (S2);

7. Sensing surface (1) according to claim 6, wherein the binary memory (21) forms binary words (22), the output signal (S2) setting to one a bit of a word (24).

8. Sensing surface (1) according to claim 6 or 7, wherein the array

(20) comprises rows (23) of detecting devices superimposed to columns (26) of detecting devices, each detecting device being independent from the other.

9. Sensing surface according to claim 8 when attached to claim 7, wherein a binary word (21) is formed from columns (26) and a binary word

(21) is formed from rows (23), the comparison of the two words (21) giving the crossing points (28) of sensors (2) generating an output signal (S2).

10. Sensing surface (1) according to claims 7 to 9, wherein sensors (2) are laid upon an electrically insulated material (24).

11. Sensing surface (1) according to claims 7 to 10, wherein the sensors (2) are integrated in a concrete screed (29).