WO2000038104A1

WO2000038104A1 - Interactive acoustic plate with improved signal processing means

Info

Publication number: WO2000038104A1
Application number: PCT/FR1999/003252
Authority: WO
Inventors: Jean-Pierre Nikolovski; Fabrice Devige; Sylvain Yon; Francis Cassagne
Original assignee: Intelligent Vibrations
Priority date: 1998-12-22
Filing date: 1999-12-22
Publication date: 2000-06-29
Also published as: AU1785300A; FR2787608B1; EP1141884A1; FR2787608A1

Abstract

The invention concerns a device for acquiring co-ordinates of interaction points of an acoustic source with the surface of a plate of finite dimensions comprising a set of sound sensors (SMC1 to SMC4) each formed by a pair of piezoelectric transducers located opposite each other on either side of the plate, the device comprising processing means for determining the co-ordinates of said interaction point by analysing the difference in propagation time of the sound waves transmitted by the source to the different sensors. The invention is characterised in that the processing means comprise associated with each sensor a particular electronic circuit comprising cascade wideband pre-amplifier means, and selective amplifying means centred on a first predetermined frequency.

Description

INTERACTIVE ACOUSTIC PLATE WITH IMPROVED SIGNAL PROCESSING MEANS

The present invention relates generally to interactive communication devices between a user and a machine

More specifically, the invention relates to a device for collecting and processing acoustic waves transmitted by a user to a plate serving as an interface with a machine, said device analyzing the propagation times of the acoustic waves in the plate in particular for measuring the X, Y coordinates of impacts on the surface of the plate

Patent WO 96/11378 discloses a device for acquiring X, Y coordinates of the point of a rigid plate to which a source occasionally transmits wave packets, by analysis of the propagation time of the waves in the plate in two X and Y directions of the plate

In this device, two pairs of transducers are associated respectively with each direction X, Y The position of the source in each direction is determined by measuring the differential of the arrival times of the wave packets on the two pairs of respective transducers of said direction The device also comprises means downstream of the transducers for processing the asymmetric Lamb mode A ₀ of the acoustic waves propagating in the plate

The source can be for example a stylus provided with wave emission means, or even a simple hard object impacting on the plate and thus generating wave packets. The device comprises a machine (for example a calculator) of processing of data that can control the transmission of signals to the user in order to allow interactive operation

An object of the invention is to improve the quality and reliability of the reception of acoustic waves in a device of the type mentioned above. Another object of the invention is also to improve the ergonomics of the device by extending the possibilities of communication between a user and the interface constituted by the plate, in particular by allowing communication without there being any impact. 'an object on the plate. Finally, the invention also aims to improve the safety of the device, in particular in the case of a plate made of a fragile material such as glass.

To achieve these goals, the invention provides a device for acquiring coordinates of points of interaction of an acoustic source with the surface of a plate of finite dimensions comprising a set of acoustic sensors each formed from a pair of transducers piezoelectric located opposite to either side of the plate, the device comprising processing means for determining the coordinates of said interaction point by analyzing the difference in propagation time of the acoustic waves emitted by the source to the various sensors, device characterized in that the processing means comprise, in association with each sensor, a respective electronic circuit comprising in cascade means for carrying out broadband preamplification, and means for selective amplification centered on a first frequency predetermined.

Preferred, but non-limiting aspects of the device according to the invention are the following:

- the sensors are four in number and the piezoelectric transducers of each sensor are piezoelectric ceramic discs bonded on either side of the plate, so that the four sensors form the vertices of a rhombus whose center constitutes the origin of the coordinates,

said first predetermined frequency is of the order of 100 kHz and said centered selective amplification means comprise a selective amplifier for detecting the fraction of energy generated by the source in the vicinity of said first predetermined frequency, the localization of a point of interaction of the source with the plate consists in extracting the ultrasonic frequency component in the vicinity of 100 kHz generated by the impact of a hard object such as the fingernail, a metal key, a ballpoint pen, a hard plastic and to determine the difference in flight times between two sensors of a first pair on the one hand and two sensors of a second pair on the other, so that the Cartesian coordinates of the impact point (x _r , y _r ) on the plate are

x _r

given by the formula: where x _M y _r

and y denote the position of the sensors relative to the center of the diamond, V the speed of the antisymmetric Lamb mode in the plate, Δt _x (respectively Δt _y ) the difference in propagation times of the wave packet generated by the impact between the sensors of the first pair (respectively of the second pair), k _x (respectively k _y ) of the coefficients equal to 1 or -1 and sign (x _r ), sign (y _r ) the signs of the coordinates x _r and y _r ,

- said electronic circuits associated with the sensors comprise in cascade a broadband preamplifier stage, a square elevator stage, a detector stage, an integrator stage, and an adaptation stage at a logic level, - said logic level is of the CMOS type ,

the electronic circuits associated with the respective sensors comprise, upstream of said selective amplification means, a bypass towards analysis means of at least one other spectral component in order to characterize the nature of the acoustic source or of the dimensions of the source, the analysis means comprise at least one selective bandpass filter centered individually on a predetermined centering frequency different from said first predetermined frequency, and the processing means comprise downstream of said electronic circuits associated with the respective sensors a programmable logic module,

- said centering frequency is located in the audible spectrum, preferably in the upper limit of the audible spectrum,

said programmable logic module comprises a logic component capable of determining whether a signal generated by the source in the plate comprises significant components both around the first predetermined frequency and around the centering frequency (s),

- Means are provided for determining the intensity of a signal transmitted by the source to the plate around the first predetermined frequency, - said intensity determination means comprise means for measuring the number in the form of a reverberation. echoes on the edges of the plate of an acoustic wave generated around the first predetermined frequency by an interaction of the source and the plate,

a 12-bit counter powered by the output of the selective amplification means centered on a first predetermined frequency originating from one of the sensors and adapted to CMOS logic is provided for measuring the reverberation;

- the impact counter is activated by a stable clock, counting being authorized as long as the envelope of successive echoes of the wave train generated in the plate in the vicinity of the first predetermined frequency remains above a determined threshold ,

- means are also provided for determining the intensity of a signal transmitted by the source to the plate around at least one centering frequency different from the first predetermined frequency, - there is also provided at least one other associated counter individually at a centering frequency and capable of accounting for the number of echoes on the edges of the plate of an acoustic wave generated around said associated centering frequency by an interaction of the source and the plate,

means are provided for determining the nature of the source as a function of the intensity of the signal transmitted from the source to the plate around the first predetermined frequency and of the centering frequency (s),

- means are provided for sending a user a luminous message via a peripheral connected to the processing means, indicating to him whether an interaction has generated an acoustic signal being within a desired reverberation range,

- according to a threshold value reached by one or more counters each associated individually with a frequency, the device is capable of triggering preventive actions for the safety of the plate, for example by activating an alarm or by triggering the closing of a metal curtain covering the plate,

the source is able to excite the plate without said plate transmitting sound waves to the ambient environment,

the source is a stylus, a hollow longitudinal main body of which comprises an electrical energy source, an electrical pulse generator capable of being powered by the electrical energy source, a piezoelectric ceramic disc capable of being excited by the pulse generator for bringing into longitudinal resonance a cavity of the main body filled with a component having a mechanical impedance comparable to that of the material constituting the main body, said cavity of the main body being closed by a pellet made of a soft material d comparable impedance such as silicone,

the electrical pulse generator is capable of continuously emitting bursts of pulses around 100 kHz and the hollow main body is made of a material resonating around this frequency, for example a plastic material, the component filling said cavity of the main body is an antifreeze liquid,

- the stylus includes a power switch to control the emission of acoustic waves, - the plate is capable of being excited by a compression wave generated by the voice of a user, and the output of the wide preamplification stage band of at least one of the electronic circuits associated with the respective sensors is derived to low-pass amplifiers, for example Bessel filters of order 6 limited to the audible band, - the output of said low-pass amplifiers is diverted to a Jack stereo output connector and then to an audio capture card,

- one or more electronic band-cut filters are inserted after the low-pass filters to compensate for the plate's own resonances at certain frequencies,

- it includes a software filter with function of reverse transfer of the acousto-electric frequency response of the plate on a given frequency band of a few kiloHertz, to process the audio signals coming from the glass plate, - one of the sensors is able to be switched into transmitter of a packet of ultrasonic waves in order to trigger a plate integrity test comprising the measurement:

• differences in propagation time of the wave packet between the different sensors, • and the reverberation of the wave packet using an impact counter, and the device comprises means for comparing said reverberation value with values reference and means for triggering preventive actions for the security of the plate, for example by activating an alarm or the closing of a metal curtain covering the plate, - the integrity test is triggered by a write instruction to a memory address, starting a general counter and an oscillator from which a logic burst and its logic inverse are capable of actuating the grids of electronic components controlling the adaptation of the logic burst to a higher voltage applied to the sensor switched to the transmitter, said higher voltage being obtained using the oscillator and a diode pump connected to reservoir capacitors,

the oscillator is a relaxation oscillator operating at around 100 kHz, said higher voltage is of the order of 60 volts and said electronic components are NMOS and PMOS transistors,

- the input stage of the electronic circuit associated with the sensor switched to the transmitter is protected from the higher voltage burst by NMOS switching transistors blocked during the burst and turned on when the sensor capable of being switched to the transmitter is in reception mode , - the plate integrity test is carried out when the device is started and just after an interaction between the source and the plate,

- additional pairs of transducers are dedicated to the integrity test,

- the pairs of additional transducers are also four in number and define a second rhombus oriented at 45 ° relative to the rhombus of the sensors, the pairs of additional transducers also participating in the measurement of coordinates of interaction points between the source and the plate,

the device constitutes a peripheral interface with a machine which receives the signals coming from the processing means, and which according to said signals controls the execution of files contained in the computer by various peripherals of known types connected to the computer such that for example without limitation a video projector, light sources, loudspeakers, a printer, or even an automation unit controlling a mechanical action such as the closing of a protective curtain, - said machine is a computer with a screen,

a pixel of coordinates on the computer screen showing an image of the plate is associated with a physical point of the coordinate plate, the conversion of the coordinates comprising the following operations: a) visual marking of the center of the sensors, b ) recording of a digital image including the origin, c) delimitation on the digital image of a working frame of center C, and calculation of the length and the height in pixels of the working frame, d) definition d '' a homothetic correspondence which gives the scale of the working frame, i.e. the length L and the height I in millimeters corresponding on the plate to the drawn working frame, e) positioning on the digital image of a cursor pointing to the center of the sensors, i.e. to the origin of the real coordinates of the plate, visually identified in a), so that any pixel of coordinates N _X q, N _yq of the working frame will be linked at its actual coordinates according to the formula :

a portion of the plate defined as a working frame equivalent to the computer screen is divisible into zones of any shape, each zone corresponding to a domain of the digital image of the plate which is stored in the computer and which is able to be activated by an interaction at a place on the plate belonging to the zone, to control the execution of different groups of programs on the computer,

- the computer is able to carry out at least one of the following operations:

• a time delay which neutralizes the detection of the acoustic waves for a given time interval after the detection of an interaction of the source and the plate, • tests of the proper functioning of a signal acquisition card, by visualizing on the computer screen the location of a physical impact on the plate as well as the value of impact counters,

• a simulation of the operation of the plate without involving the plate and the acquisition card, by triggering the sequences using the computer mouse which is positioned and operated on the desired areas of the digital image the frame of the plate,

• a global backup of the computer files linked to the functioning of the plate (in particular the files constituting the sequences associated with each of the fields of the digital image of the plate) according to the following rules: a) all the files forming a sequence are saved in a directory bearing the name of the domain with which this sequence is associated; b) all the domain directories are in a single directory containing in addition the source file,.

• help for a user in the form of a text or an image explaining all the possible actions at a given instant when a sequence is running or describing the means of launching a sequence when the device is in state waiting,

• a statistical module to exploit the information collected by the computer, and indicating in particular the number of interactions having been carried out during a given period with each zone of the plate's working environment, - additional means are provided allowing the triggering, for example, at night of the lighting of the plate or of an area of the plate following an impact on any or predetermined part of the working frame of the plate,

the constituent transducers of each are connected to a resistance bridge associated with the sensor before attacking the wideband preamplifiers of the electronic circuits associated with the sensor, - the plate is made of glass.

Other aspects, aims and advantages of the present invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example and made with reference to the accompanying drawings, in which :

FIG. 1 is a perspective view of a display case equipped with sensors and operating according to the principle described in patent WO 96/11378,

FIG. 2 is a schematic sectional view showing two types of impact perpendicular to the display case operated with the same elongated object, here a key, held differently,

- Figure 3a is a block diagram of the four identical analog channels for processing the signals supplied by the receivers of the display cabinet of Figure 1, - Figure 3b is a block diagram of the analog circuits for processing one of the four channels of Figure 3a,

- Figure 4 is a block diagram of connectors, analog, digital circuits for processing signals from different electrical nodes of the four channels of Figure 3a, - Figure 5 is a block diagram of part of the internal architecture of the programmed component "wavepro3" in figure 4,

- Figure 6 is a block diagram of the audio low-pass filtering circuits of the signals from the preamplifier stages of two channels of Figure 3a, - Figure 7 is a schematic view in axial section of a stylus according to an embodiment preferred of the invention,

FIG. 8 represents a schematic view of the electrical balancing circuit of one of the four sensors of the display case of FIG. 1,

FIG. 9 is a block diagram of circuits for generating pulses in burst mode at 60V for carrying out a showcase integrity test, FIG. 10 is a block diagram of part of the internal architecture of the programmed component "wavepro3" intended for the generation of control signals of the transistors of the pulse generator of FIG. 9,

FIG. 11 is a schematic view of a display case showing a doubling of the number of sensors, compared to the configuration of FIG. 1,

- Figure 12 is a schematic view of the window of Figure 1, on which we superimposed and coincide the limits of an image scanned in 640 x 480 pixels of a part of the window, allowing to achieve a correspondence between the actual coordinates of an impact point and the pixel coordinates of the digital image of this same point.

Referring first to Figure 1, there is shown a plate 10 such as that described in patent WO 96/11378. This plate comprises four pairs SMC1, SMC2, SMC3, SMC4 of piezoelectric transducers which each constitute an acoustic sensor, the two transducers of each pair being fixed vis-à-vis on the two opposite faces of the plate (for example by collage) to collect the acoustic waves circulating in the plate.

An orthogonal coordinate system X, Y of origin O is associated with the plate 10, the center of the plate possibly coinciding with the origin O of the coordinate system. The SMC1 and SMC2 sensors are located on the X axis, symmetrically opposite to the origin of the coordinate system. They have the respective coordinates (-XM, 0) and (XM, 0). The SMC3 and SMC4 sensors are located on the Y axis, symmetrically opposite to the origin of the coordinate system. Their respective coordinates are (O, -Y _M ) and (O, Y _M ).

Acoustic waves can in particular be generated by the impact of an object on the plate. As will be seen, they can also be generated by other forms of excitation of the plate by a user. The plate is made of a rigid material constituting a good isotropic acoustic conductor around 100 kHz, such as glass. This plate can be flat as in the present embodiment where it constitutes a glass display case, but can also be defined by a curved surface.

The Applicant has observed that the acoustic frequency spectrum of the antisymmetric Lamb mode Ao generated by an object striking the isotropic display case, which extends from a few tens of Hertz to several hundred kiloHertz, has properties which depend on the nature and the dimensions of the object used to communicate with the display case.

Thus, the softer the object striking the display case, the less the spectrum contains components towards the high frequencies. So if we consider for example the finger of the hand, the spectrum will be different depending on whether we strike with the skin or the fingernail. The difference is also remarkable in the audible spectrum.

Furthermore, if we consider a hard object such as a metal key, the acoustic spectrum will also depend on how we strike the window, as we will see with reference to the example in Figure 2.

When the window is struck with a slender C1 key oriented perpendicular to the surface of the window, the acoustic spectrum of the waves generated in the window contains fewer components towards the high frequencies than if the shock were produced by tilting the key ( key C2). The reason for this difference is related to the duration of the impact. The longer the impact, the lower the spectrum generated. When the elongated key that strikes the window is perpendicular to it, the duration of the impact is greater than if one strikes while holding the inclined key. Indeed, the duration of the impact of an object is linked to the transit time of the momentum in the object during percussion. A slender object striking the window perpendicularly will be associated with a transit time of a longitudinal acoustic wave back propagating along the striking object. If this object hits the window with a perpendicular approach but is kept tilted (key C2), the reaction of the window remains perpendicular and the amount of movement which backs up in the object by reaction of the window lasts the time that the acoustic wave takes to reach the limits of the object in the direction perpendicular to the window, time which is shorter than in the case of a key oriented perpendicular to the display case (key C1).

Thus, it is possible to deduce from the spectrum of the acoustic frequencies generated by the impact of the information on the object used by a user to communicate with the display case. We will see below that the Applicant has developed means to exploit such information to achieve the aims of the invention.

The operation of the interactive display case of WO 96/11378 involves the detection of the antisymmetric Lamb mode Ao at ultrasonic frequencies sufficiently high to be able to obtain good accuracy in the detection of acoustic waves around 100 kHz. As shown in FIG. 3a, there is associated with each sensor of the display case, which delivers a respective signal AOPX-, AOPX +, AOPY-, AOPY +, a respective electronic detection circuit D1, D2, D3 and D4. The circuits D1 to D4 are equivalent.

Figure 3b details the main elements of circuit D1. This circuit performs broadband preamplification, followed by selective amplification centered on 100 kHz using a selective amplifier IC2G $ 2 to detect the fraction of energy generated by an impact in the vicinity of this frequency.

This selective amplification is important, because the Applicant has observed that in general this fraction is very small compared to the recoverable fraction in the audible spectrum in the case of a signal generated by an impact on the display case.

Following this selective amplification centered on 100 kHz, the signal is squared by the circuit referenced IC3, then a peak detection is carried out by the circuit referenced IC4G $ 1 and IC4G $ 2. Following this peak detection, the signal is integrated using the IC5 operational amplifier circuit passing the output of this amplifier from a positive saturated level obtained using resistance bridges on the inverting inputs, to a negative saturated level when a wave packet is detected.

This transition from a positive saturated level to a negative saturated level on the gate of the PMOS transistor T1 has the effect of turning it on as soon as the gate voltage drops below 3.5 volts.

The respective outputs XMOINS, XPLUS, YMOINS and YPLUS of circuits D1, D2, D3 and D4 are connected to a processing computer type computer, via components which will be described later in this text.

We will now detail various aspects of the invention which make it possible to ensure reliable and ergonomic operational operation of a display case operating according to the principle described in patent WO 96/11378, in particular in an environment comprising acoustic disturbances. For this purpose, we base ourselves on the example of a shop window equipped with sensors and to which a user transmits acoustic signals to interact with a computer connected to the sensors.

According to a first aspect, an operational problem linked to the reception of the acoustic waves generated around 100 kHz is the presence of terrestrial noise, for example from micro-vibrations of the subsoil, in this frequency domain. These noises which propagate on the surface of the earth can be transmitted to the showcase and excite the piezoelectric transducers, which can then generate an untimely detection of waves around 100 KHz and disturb the functioning of the showcase. Terrestrial noises, however, have an acoustic spectrum different from the spectrum generated by a user striking the window with his nail or a hard object to interact with a computer connected to the window. Such percussions would indeed emit waves both around 100 kHz and in the audible spectrum. Terrestrial noise presents very few spectral components in the audible domain (defined in this text as low frequency domain). To prevent the display case from being activated by terrestrial noise, a solution therefore consists in carrying out a frequency analysis of the detected signal, in order to verify that a signal detected around 100 kHz by circuits D1 to D4 also contains frequencies in the audible range. This can be done in a known manner from a digitization of the signal and appropriate processing, for example the study of the discrete Fourier transform of the digitized signal.

More simply and more economically, a preferred solution according to the invention consists in carrying out a selective amplification, for example in the vicinity of 10 kHz and preferably in the upper limit of the audible spectrum, on one of the four channels coming from the respective sensors of the showcase, after a broadband preamplification of the signal.

FIG. 4 thus shows that the output BFX- of the broadband preamplification stage of the selective amplifier of the circuit D1 of FIG. 3b is adapted to the CMOS logic by switching a PMOS transistor T5. This transistor can then validate a control flip-flop to determine if the detected signal contains low frequencies

In the preferred embodiment of FIG. 4, if a low frequency component from the sensor SMC1 is detected, T5 activates the clock input of a LF counter of a programmable logic component WAVEPRO3, while the signals XMOINS, XPLUS , YMOINS and YPLUS centered on 100 KHz and coming from circuits D1 to D4 are also transmitted to the WAVEPRO3 component in which these signals trigger, as will be detailed later in this text, respective flip-flops. In the WAVEPRO3 component, the structure and operation of which will be explained with reference to FIG. 5, a logical AND function identifies the signals carrying energy both in the low frequency spectrum and in the spectrum close to 100 kHz, and thus makes it possible to to discriminate against untimely tripping of the display case due to noises of terrestrial origin. On the other hand, according to a second aspect which may or may not be linked to the above aspect, the proper functioning of the display case also involves quantify the amplitude of a spectral component of an acoustic wave circulating in the display case.

Indeed, it is possible that an impact on the showcase is just strong enough to trigger only some of the respective flip-flops associated with the four showcase sensors. In this case, determining the coordinates of the point of impact on the display case is impossible.

It is also possible that a relatively weak signal generated by an impact on the showcase triggers in the component WAVEPRO3 the four rockers of the four showcase sensors, but that due to the difference in distance between two sensors associated with the same direction and from the quadratic integration which is carried out on the signals delivered by the sensors by the respective circuits D1 to D4 after the selective amplification around 100 kHz, there appears a significant asymmetry in the amplitude of the signals of the pair (XMOINS, XPLUS ) or of the pair (YMOINS, YPLUS). In this case, determining the impact coordinates is also problematic and the operation of the display case is not correct.

Conversely, a violent impact on the window made for example with a metallic object can generate faster modes than the Ao mode, in particular the symmetrical mode So, and this in sufficient proportion for the flip-flops of the device to switch on this. symmetrical mode for which they are not intended. In this case, the normal operation of the display case is also disturbed.

It is therefore necessary to have quantitative data on the amplitude of the acoustic signals circulating in the display case in order to determine whether the detected signal is within an acceptable amplitude range.

A preferred method according to the invention for quantifying the amplitude associated with an impact in the display case consists in measuring the reverberation of the signal in the display case following the impact. Here we define reverberation as the time it takes for the signal level to drop back below a determined amplitude threshold following an impact. In the particular case of a shop window, this time depends first of all on the intensity of the shock but also on the size of the shop window and the absorbent effect of the holding joints of the shop window. With a constant window size, this time can therefore vary from one window to another. For a given showcase, the minimum and maximum reverberations surrounding the correct operation of the showcase will be calibrated during the installation of the showcase and the sensors.

To quantify the reverberation associated with acoustic waves following an impact on the display case, one of the respective output signals AMPX-, AMPX +, AMPY-, AMPY + of the selective filters centered on 100 KHz is adapted to CMOS logic using of an NMOS transistor T6 as shown in FIG. 4, then sent to the clock input CLKi of a 12 bit impact counter of the component WAVEPRO3.

This counter allows to count the number of crossings, by a signal around 100 KHz, of a predetermined amplitude threshold.

It should be noted that the acoustic signals generated by an impact on the display case can be detected for the first time by the four acoustic sensors and then rebound on the walls of the display case which constitute the borders of a full glass cavity in which the waves are trapped. The sensors can therefore also detect successive passages of reflected wave trains from the same impact, if the latter transmits a large amount of energy to the display case.

The CLKi counter makes it possible to account for a given impact the total number of threshold crossings, whether they originate from a first incident wave train or from reflected wave trains, and thus establish the total amount of energy associated with impact. The reverberation time is measured by the 12-bit counter in 10 μs steps.

A variant of this method of measuring reverberation at 100 kHz consists in controlling the clock input of the impact meter CLKi using a stable clock at 100 kHz, counting being authorized as long as the envelope of the successive echoes of the generated wave train in the plate in the vicinity of 100 kHz remains above a determined threshold.

The system according to the invention thus comprises an impact counter which makes it possible to quantify the energy associated with a shock. This information transmitted to the computer to which the WAVEPRO3 component is connected can then be used by this computer to generate a signal relating to the quality of the impact intended for the user.

Thus it is possible for example to use a peripheral connected to the computer to send a light signal to a user to indicate to him if an impact is within a desired operating range: green light for an acceptable shock, light red for too weak or too strong shock.

According to the invention, it may also be advantageous to simultaneously quantify the energy associated with an impact while performing a spectral analysis as described above according to the first aspect mentioned of the invention.

In fact, it has been said that impacts generated by objects of different hardness produce acoustic waves in the window with a different spectral distribution. By quantifying the energies associated with two or more given frequencies, we can determine the nature of the material used to strike the display case, which can be advantageous, for example, in terms of security (such an arrangement would indeed make it possible to trigger a preventive protective action. , for example by activating an alarm or by triggering the closure of a metal curtain covering the display case, in the event of a violent impact with a hard object).

Referring to Figure 5 which details the architecture of the component

WAVEPRO3, a preferred embodiment of the invention consists in using two impact counters for this, one activated by the selective filters at 100 KHz

(CLKi counter), the other activated by a low frequency filter at 10 KHz (LF counter).

The WAVEPRO3 component works according to the following logic: • Four flip-flops FF1 to FF4 associated with the four respective sensors of the showcase can be activated at their clock input by a respective signal XMOINS, XPLUS, YMOINS, YPLUS from a percussion on the showcase, • SignX, SignY of coordinates of the point of impact are then determined by flip-flops FF5 and FF6. The flip-flops FF1 to FF6 are reset by a write order to a given address on the card,

• A 16-bit PROCESS COUNTER general counter counting at the frequency of a 1 MHz ck clock is activated by detection of a percussion by a sensor. This general counter governs the chronological stages of acquiring the coordinates.

In particular, this counter sends on an ISA data transmission bus to the computer a WrHF message of signal presence in the 100 KHz range, and this 49.152 ms after impact. This delay gives the BF and CLKi impact counters time to quantify the reverberation at 100 KHz as well as at 10 KHz following an impact. The WrHF message remains active for 2 ¹⁴ clock periods, or 16.38 ms.

• When this WrHF message is perceived by the processing computer, the data present on the three-state buses associated with memory addresses are read sequentially by the computer and transformed into pixel coordinates on a digitized image of the display case according to the protocol which will be described later,

• The specific programs contained in the computer and the steps of which will also be explained below also verify the state of a WrBF bit corresponding to the input of the BF impact counter. Computer programs thus have WrHF and WrBF signals and can determine whether a WrHF trigger corresponds to a terrestrial signal or not, by the WrBF signal,

• According to a particular embodiment of the invention, when a shock causes WrBF to switch and WrHF does not switch in a predefined time interval after the switching of WrBF, this time interval can be around 70 ms, computer software deduces that the user uses an object whose impact on the window generates low frequency waves, but that this object is too soft to communicate effectively with the showcase. The computer can then send, via a device such as a projector or monitor, a message to the user recommending the use of a harder object.

According to another aspect which can be considered independently or not of the aspects mentioned above, the invention proposes to improve the ergonomics and the comfort of use of the display case by extending the communication between the user and the display case to modes interactivity not using percussion. It may indeed be particularly advantageous to suppress the percussion with a hard object on the display case, such percussions not being tolerable for example for a trader who does not have enough space to work in peace at a sufficient distance from the display case interactive.

Thus, according to a first alternative embodiment, the surface of the display case is used with profit as a microphonic membrane. Indeed, if an object striking the window injects energy into it at the place of impact, the surface receiving the energy is however very small, typically of the order of a few tens of square microns for a key. A person speaking in front of the showcase emits towards it a compression wave which will mechanically excite a much larger surface of the glass plate of the showcase.

The excitation of the showcase then taking place on a considerably larger surface than in the case of a specific impact, it also results in a signal detectable by the four sensors of the showcase. The anti-symmetrical mounting of the pairs of piezoelectric transducers which is described in patent WO 96/11378 is entirely suitable for detecting the displacement of such a wave. In the case of audible frequencies, for example at 1 KHz, the wavelength emitted is large compared to the thickness of the display case and the latter behaves like a membrane. In addition, the piezoelectric discs which constitute the acoustic sensors of the device have an area of the order of a few square centimeters which is much smaller than the excited area of the display case. The more the frequency of the emitted wave decreases, the more the amplitude of the displacement of this membrane and therefore of the signal increases to get closer to its fundamental resonance mode which is around a few hundred Hertz for a window of a few square meters.

A user can thus communicate with the computer by speaking and vibrating the display case. As shown in FIG. 6, the output of at least one broadband amplifier from the respective circuits D1 to D4, for example BFX- or BFX +, is for this purpose diverted to low-pass amplifiers, for example implementing a filter of Bessel of order 6, limited to the audible band and intended to adapt in amplitude and electrical impedance the signal in order to be able to send it via a connector of type Jack towards an entry of sound card.

The signal can also undergo in certain cases a convolution by a multipolar filter having for goal to compensate for the selectivity of the showcase at certain frequencies and to obtain a regular response of the showcase which then acts like a microphone on a frequency band of a few kiloHertz.

It should be noted here that from a practical point of view, the insertion of a filter at the level of the electronic acquisition card is only of interest when a large number of display cases of the same dimensions have to be produced. More conveniently, to adapt the device to various dimensions of plates, a software filter integrated in the computer is used.

At the audible frequencies in which we are interested here, the showcases are rectangular membranes satisfying rigid boundary conditions: the mechanical displacement normal to the glass plate is zero at the joint which maintains the edge of the showcase. It is therefore sufficient to enter the dimensions of the rigid frame for holding the display case to determine the transverse resonance frequencies of the glass plate and therefore the transfer function of the glass. A reverse transfer function, known as a Wiener filter, is then calculated on a frequency band of a few kiloHertz and applied to any audio recording coming from the glass plate.

It should be noted that stereo recordings are possible by associating with two sensors, for example the sensors SMC1 and SMC2, two respective audio inputs. Indeed, the speed of the asymmetric waves in a glass plate is about 3400 m / s, or 10 times faster than the speed of sound in air. In other words, SMC1 and SMC2 sensors, for example 1.5 meters apart, are equivalent in terms of time of flight difference to microphones spaced 15 cm in the air. The embodiment of the invention in FIG. 6 thus represents a circuit processing two preamplified signals BFX- and BFX +.

As a source of extensive mechanical excitation, and low frequency, the voice does not point to a specific place in the window. However, it advantageously supplements the ergonomics of a display case with which the detection and processing means described above are associated to determine the X, Y coordinates of an impact on the display case.

The variant described above thus provides an advantageous complement, a specific voice signal being able to trigger an action such as the supply of a light source to illuminate the display case or to allow a voice message to be left. In a second variant which also makes it possible to communicate with the window without percussion, and which in turn makes it possible to determine the X, Y coordinates of a point of the window chosen by a user, it is possible to provide an ultrasonic wave generator that the user can move silently across the surface of the display case. As shown in FIG. 7, such an ultrasonic generator can be produced in the form of a stylus 100 enclosing in a main body 101 longitudinal hollow a source of electrical energy 102 located in the upper part of the body and constituted for example by batteries, a continuous generator of bursts of pulses and a piezoelectric disc 104 made of ceramic which, excited by the generator pulses when the latter is powered by the batteries, resonates a cavity 105 in the lower part of the body made of a plastic material and filled with an antifreeze liquid 106 having a mechanical impedance comparable to that of the material constituting the cavity 105.

Typically, a cylindrical polycarbonate cavity about 7 mm high and about 5 mm in diameter filled with a standard motor antifreeze is suitable for generating pulses of the order of 100 KHz.

To transmit this mechanical resonance to the window, the lower end of the cavity 105 opposite the ceramic 104 is mechanically coupled to a pad 107, made of a material such as silicone, bonded to this closed end of the cavity and the convex surface provides a large mechanical coupling surface with the display case 10.

The mechanical resonance of the cavity 105 is caused by exciting the piezoelectric pad 104 by an electric pulse comprising for example a burst of 5 square pulses of 48 Volts amplitude, at the resonance frequency of the cavity 105, ie 100 KHz . Acoustic waves are thus transmitted to the display case around 100 KHz when the stylus is kept in contact with the display case by the user at a desired point thereof.

It is also possible to provide the stylus with a switch to allow the user to control the supply of batteries 102 and therefore the emission of acoustic waves.

It will be noted that this particular embodiment must preferably be used with a display case that is correctly isolated from ground noise, for example a window mounted on elastomer supports or a window of a glazed door. Indeed, a stylus such as that described above is not suitable for transmit only around the resonant frequency of the cavity and does not emit any low frequency wave.

According to another advantageous aspect which can be considered independently of the aspects mentioned above, the invention also proposes an improvement aiming at the rejection of the symmetrical modes of propagation of the acoustic waves in the window, in particular the So mode.

The showcase of patent WO 96/11378 already proposes to eliminate these symmetrical modes of equipping each sensor with a pair of transducers mounted opposite one another on either side of the glass plate of the showcase according to a particular mode.

However, a misalignment 10 of the contact surfaces of the transducers with the display case 10 (due to the parallax for example) alters the effectiveness of this arrangement. An advantageous solution to remedy this is to use discs whose diameter is large as piezoelectric transducers in the face of the potential overlap error of the discs.

As shown in FIG. 8, a difference in thickness of the adhesive layers 201, 202 used to fix the respective transducers PZT1,

PZT2 of the pair on the display case can also disturb the rejection of the symmetrical modes. An advantageous solution according to the invention is to implement a bridge of variable resistances, as shown in FIG. 8, to adjust the relative gain of the two piezoelectric discs of the same sensor.

Another characteristic of the invention relates to the addition of a functionality making it possible to check the state of the plate constituting the interface (integrity of the glass plate, of the bonding and of the state of electrical connection of the piezo sensors. electric).

For this we switch one of the four piezoelectric receivers

(for example Yplus) as a transmitter which is excited by an electrical pulse in burst mode at 100 kHz. The propagation times tγ _{+ γ are then} measured. and tγ ₊ χ.- tγ ₊ χ ₊ (t _a being the reception time differential of sensors a and b), as well as the reverberation (state of the impact counter at 100 kHz) and we compares these values with reference values that were recorded when the display case was installed. In the event of a deviation above a determined threshold, an alarm can be triggered or a protective curtain lowered via the computer. From a practical point of view, the integrity of the display case is checked when the device is started and then each time a multimedia sequence is launched by the computer, i.e. just after a shock on showcase.

For this, a relaxation oscillator constructed using the operational amplifier IC22 of FIG. 9 supplies a diode and capacitor pump C123, C124, C127, C128, C129, C130, D2S1, D2S2, D2S3, allowing obtain a voltage of 60 Volts across the terminals of the reservoir capacitors C126 and C131. The value of the reservoir capacitors is large compared to the value of the static capacities of ferroelectric ceramics. A voltage close to 60 volts can thus be applied to the terminals of one of the pairs of piezoelectric transducers in the form of a 100 kHz burst obtained by a sequence of specific commands from the transistors T7, T13, T8, T14, T12 , T9, T10, T11, T15 from the "wavepro3" component. To trigger the integrity test, the computer sends a write order to the address of the +1 card. (Example, if the card address is H300, a write order is sent to address H301).

This write order is manifested by a change of state of the bits of the address bus as well as of the bits WRB, AEN of the ISA bus, as shown in FIG. 10. These bits arrive at the input of the "wavepro3" component which decodes these states and activates an internal bit serving as clock input to the flip-flop FF7.

Still with reference to FIG. 10, writing to the address of the +1 card locks a flip-flop FF7 in state 1, which validates the starting of the General counter "Process counter". Combinatorial logic on the output bits of the general counter makes it possible to define the duration of the pulse electric in burst mode which we will apply to the pair of "Yplus" transducers.

For example, a logical AND on the last ten inverted most significant bits of the general counter remains in the high state during the first sixty four microseconds of the general counter. The output of this logical AND called "timburs" validates the start-up for 64 μs of a 100 kHz relaxation oscillator built from the external components R130, R129, C159 and the gates INV1 and NAND1 and creates a burst of 7 periods in HCMOS logic. This burst called "7F100" feeds the gates of the NMOS transistors T7 and T13 of FIG. 9, these transistors controlling themselves by turning on the passing state of the PMOS T9 and T12.

With reference to FIGS. 9 and 10, the logic inverse SINK of this burst feeds the gates of the NMOS transistors T8 and T14 responsible for absorbing the discharge current of the pair of transducers "Yplus". A resistor R95 makes it possible to prevent the leakage current from the PMOS transistors T9 and T10 from charging the pair of "Yplus" piezoelectric transducers.

To prevent the 60 Volts burst from damaging the amplifiers of the analog channel "Yplus", the NMOS transistors T10 and T11 are kept in the off state (high impedance) and T15 in the on state (low impedance ) for the duration of the pulse.

It suffices to apply the signal "timburs" on the gate of transistor T15 and its logic reverse timbursb, which switches to the high level not at the 64th clock period ck but at the 65th (thanks to a NAND function on bit Q6 combined with the output of an AND on bits Q0 to Q5 inverted). This makes it possible in fact to keep the transistors T10 and T11 blocked for an additional clock period ck compared to the duration of the burst, and avoids the high voltage glitchs at the input of the amplifying channel YPLUS.

Once the electrical excitation of the pair of "Yplus" transducers has been carried out, the device waits for the arrival of the wave packets on the other three channels and proceeds to a conventional measurement of the time differences tγ ₊ γ. and (tγ ₊ χ.-t _γ + χ ₊ ) and the relaxation time at 100 kHz in the glass plate. Then a conventional general reset order is sent to the address of the card in order to put the device back on standby for a mechanical shock.

The process which has just been described is a preferred process of the present invention since it avoids sticking additional piezoelectric transducers dedicated to the task of testing the integrity of the display case. It is also possible to stick transducers dedicated to this test on the display case.

In particular, it is possible to consider sticking transducers in the lower right and left corners of the display case, which leads to greater coverage of the surface of the display case during the integrity test. In this case the excitation of the transducers is done sequentially, and their identification is done via a decoding of a value of the Data Bus assigned to each of the transducers. Doubling the number of pairs of transducers thus not only makes it possible to better cover the surface of the display case during an integrity test, but also to improve the accuracy of the measurement in the corners of the display case. When this option is chosen, the four pairs of additional transducers also form a diamond angularly oriented at 45 ° from the first as shown in FIG. 11. A simple mathematical transformation makes it possible to carry out the changes of references.

The system according to the invention comprises, as has been said, a computer which receives the signals from the electrical processing circuits. The computer can, as a function of these signals, control the execution of files contained in the computer by various peripherals of known types connected to the computer such as for example and without limitation, a video projector, light sources , speakers, a printer or even an automation unit controlling a mechanical action such as closing a window curtain. The window then constitutes an interface, an "active" part of which will be called "working environment" can be activated in a specific way by a punctual shock in a specific location (X, Y), different areas of the window work frame allowing, as we will see, to execute different groups of respective programs on the computer.

To ensure such a functioning of the system, in which the working frame of the display case is equivalent to the computer screen on which one can click in different places with a mouse, it is necessary to associate each point (X, Y) from the display case one pixel from the computer screen.

For this, the following operations are carried out which are described with reference to FIG. 12. With reference to this figure, the following symbols will be used:

X _r = Cartesian real X coordinate of the impact calculated via the position of the sensors with respect to the origin O of the coordinate system X, Y, Y _r = Cartesian real Y coordinate of the impact calculated via the position of the sensors with respect to the origin O of the reference X, Y,

L = actual width of the working frame in mm,

I = actual height of the working frame in mm, N _c χ, N _cY : pixel coordinates of the center of the working frame,

C = center of the working environment,

O = origin of the real coordinates.

• We visually mark the origin O of the X, Y coordinates of the display case, for example by applying a colored adhesive patch or a piece of adhesive tape at this point,

• a digital image of 640 x 480 pixels including the origin O is produced. This digital image can be produced by taking a photo with a digital camera or by scanning a photo of the display case,

• on the computer, we then draw the desired work environment corresponding to the area of the display case that can be activated by a user, • we deduce the dimension of the working frame in pixels. In the example of FIG. 11, the length L in the direction X corresponds to 309 pixels (Nmaxx) and the height I in the direction Y corresponds to 257 pixels (Nm _aX y),

• a homotetic correspondence is then established which gives the scale of the working environment. In the example of FIG. 11, 309 pixels correspond to 1400 mm which is the physical length L of the working frame along X and the width I of this frame which physically of 1164 mm corresponds to 257 pixels,

• on the computer, we point the cursor at point O which is the origin of the actual X, Y coordinates of the display case and which, as we said, has been visually identified and we deduce the coordinates of this point in pixels. The coordinates of O can be positive or negative, the point O not necessarily being included in the working framework of the showcase, however this point O origin of the real coordinates of the points of the showcase must appear on the digital image of the showcase,

Considering then any point q of the working framework and belonging to an area i, the coordinates of q are Nχ _q N _Yq as defined by the following formulas:

kx and k _Y are coefficients equal to +/- 1. They are used to change the orientation of the axes. In particular, a possible error when wiring the sensors consists of making a mistake on the orientation of the X axis. In the guide of the passer-by in the street, click on the positive X (either to the right of the window when the we look at the window while being in the street) corresponds to a click to the left of the window when we look at the window inside the store. By definition, the positive X axis is that of the street, that is to the right of the window when looking at the window while standing from the street.

In the above formulas, Δtx and Λty correspond respectively to the differential in the arrival time of an acoustic wave generated by a shock at point q by the two respective sensors of direction X and direction Y, and V is the speed of the acoustic waves in the plate. Note that the four acoustic sensors are not necessarily placed at the edge of the display case.

Once this correspondence has been established, it is then possible to define a partition of the display case's working frame into a plurality of zones, each zone corresponding to an area of the digital image of the display case which is stored on the computer and which commands the execution of a predetermined sequence of programs by the various peripherals mentioned above. It is obviously possible to organize at will the sequences associated with a given domain of the digital image. It is thus possible to associate with a given domain an executable program or several executable programs which are triggered successively or simultaneously and which can implement one or more peripherals. According to an advantageous characteristic of the invention which improves its ergonomics and the reliability of its operation, one can also provide in the software means of the computer a timer which neutralizes the detection of the acoustic waves during a given time interval after the detection of 'an impact. Such a timer makes it possible to avoid detecting two successive impacts that can be generated by the rebound of an object used to activate the display case.

The reception neutralization period may for example be of the order of 300 milliseconds, the Applicant having observed that such physical rebounds of a hard object on a glass plate generally occur 50 to 100 milliseconds after an initial impact. According to the invention, it is also possible to provide tests of the proper functioning of the system:

• It is thus possible to verify the operation of the signal acquisition card, by visualizing on the computer screen the location of a physical impact on the display case. This makes it possible to verify the correct geometrical positioning of the point corresponding to the point of impact on the digital image of the display case and thus to detect in particular any offset or any error in scale.

• In addition, it is also possible to display a parameter quantifying the energy associated with the impact in order to carry out a calibration to determine the desired range of impact intensity corresponding to proper operation of the display case.

It is also possible to carry out simulations of the operation of the display case without involving the display case and the acquisition card, by triggering the sequences using the computer mouse which is positioned and operated on the fields. desired digital image of the window frame.

Other advantageous functionalities of the system according to the invention include: • the global backup of the files linked to the operation of the showcase (in particular files constituting the sequences associated with each of the fields of the digital image of the showcase). It is indeed necessary, once the window is completely defined, to make a global backup of all the data in a specific directory on the computer disk according to the following rules:

> All the files forming a sequence are saved in a directory bearing the name of the domain with which this sequence is associated,

> All the domain directories are in a single directory containing in addition the source file (.VIS) and the other necessary files (statistics file which we will discuss below, default image). This directory has the same name as the source file.

Thus, a showcase is perfectly defined by the knowledge of a unique directory (ease of backup, transfer and recovery). The content of this unique directory can then be transferred to the directory of the hard drive of a computer identified on a network, the connection to the remote computer can be made via a modem.

Help can also be provided to a user in the form of a text or an image explaining all the possible actions at a given instant when a sequence is running or describing the means of launching a sequence when the system is in standby state. This text or this image can appear at the bottom of the computer screen and can also be projected by a peripheral in the form of a bright image on a screen associated with the showcase or on a part of the showcase it -even. It is also possible to display, still on the computer screen and / or near or on the display case, a default image to show a potential user a practical example of use of the system, consisting for example of a photo of a character using the display case.

It is also possible to provide a statistical module for exploiting the information collected by the computer, and indicating in particular the number of impacts having been received during a given period by each zone of the work frame of the display case. In the context of a commercial operation for example, this would inform the owner of the showcase about the interest aroused by the different areas of the showcase. This monitoring can be carried out live by constantly displaying the number of impacts received by each zone.

It is also possible, for example in the context of a shop window, to switch off all the lighting parts of the shop window at night and program the computer to then trigger the lighting of the shop window following an impact on any part. or predetermined of the work environment of the showcase. Finally, it is possible to extend the functionalities of the system by providing, for example, additional means of communication with the user, such as for example a keyboard by which a user can enter a certain amount of information to register, to request additional information or to identify yourself following a registration and then request reserved information.

Such an identification system is an effective alternative to the means of recording speech mentioned above when the surrounding noise does not allow the implementation of such sound recording means.

Claims

1. Device for acquiring coordinates of points of interaction of an acoustic source with the surface of a plate (10) of finite dimensions comprising a set of acoustic sensors (SMC1 to SMC4) each formed from a pair of transducers piezoelectric (PZT1, PZT2) located opposite each side of the plate, the device comprising processing means for determining the coordinates of said interaction point by analyzing the time difference of propagation of the acoustic waves emitted by the source towards the various sensors, device characterized in that the processing means comprise in association with each sensor (SMC1 to SMC4) a respective electronic circuit (D1 to D4) comprising in cascade means for performing a broadband preamplification, and selective amplification means (IC2G $ 2) centered on a first predetermined frequency.

2. Device according to claim 1 characterized in that the sensors are four in number and the piezoelectric transducers of each sensor are piezoelectric ceramic discs bonded on either side of the plate, so that the four sensors form on the plate the vertices of a rhombus whose center constitutes the origin (O) of the coordinates.

3. Device according to claim 1 or 2, characterized in that said first predetermined frequency is of the order of 100 kHz and said means of centered selective amplification comprise a selective amplifier (IC2G $ 2) for detecting the fraction of energy generated by the source in the vicinity of said first predetermined frequency.

4. Device according to claims 2 and 3, characterized in that the location of a source interaction point with the plate consists in extracting the ultrasonic frequency component in the vicinity of 100 kHz generated by the impact of an object hard such as the fingernail, a metal key, a ballpoint pen, a hard plastic and to determine the difference in flight times between two sensors of a first pair (SMC1, SMC2) on the one hand and two sensors of a second pair (SMC3, SMC4) on the other hand, so that the Cartesian coordinates of the point of impact (x _rι yr) on the plate are given by the formula

where x _M and yM denote the position

sensors relative to the center (O) of the rhombus, V ia speed of the

Antisymmetric lamb in the plate, Δt _x (respectively Δt _y ) the difference in the propagation times of the wave packet generated by the impact between the sensors of the first pair (SMC1, SMC2) (respectively of the second pair (SMC3, SMC4)), k _x (respectively k _y ) coefficients equal to 1 or -1 and sign (x _r ), sign (y _r ) the signs of the coordinates x _r and y _r .

5. Device according to one of claims 1 to 4, characterized in that said electronic circuits (D1 to D4) associated with the respective sensors (SMC1 to SMC4) cascadingly comprise a broadband preamplifier stage, a square riser stage (IC3 ), a peak detector stage (IC4G $ 1, IC4G $ 2), an integrator stage (IC5), and an adaptation stage at a logic level.

6. Device according to claim 5, characterized in that said logic level is of the CMOS type.

7. Device according to one of claims 1 to 6, characterized in that the electronic circuits (D1 to D4) associated with the respective sensors comprise, upstream of said selective amplification means, a bypass (BFX-) towards analysis means at least one other spectral component in order to characterize the nature of the acoustic source or the dimensions of the source.

8. Device according to claim 7, characterized in that the analysis means comprise at least one selective bandpass filter individually centered on a predetermined centering frequency different from said first predetermined frequency, and the processing means comprise downstream of said electronic circuits associated with the respective sensors a programmable logic module (Wavepro3)

9. Device according to claim 8, characterized in that said centering frequency is located in the audible spectrum, preferably in the upper limit of the audible spectrum.

10. Device according to claim 8 or 9, characterized in that said programmable logic module comprises a logic component capable of determining whether a signal generated by the source in the plate comprises significant components both around the first predetermined frequency and around of the centering frequency (s).

11. Device according to one of the preceding claims, characterized in that means are provided for determining the intensity of a signal transmitted by the source to the plate around the first predetermined frequency.

12. Device according to claim 11, characterized in that said intensity determination means comprise means for measuring in the form of a reverberation the number of echoes on the edges of the plate of an acoustic wave generated around the first frequency predetermined by an interaction of the source and the plate.

13. Device according to claim 12, characterized in that it is intended to measure the reverberation a 12-bit counter (CLKi) supplied by the output (AMPX-, AMPX +, AMPY-, AMPY +) of the selective amplification means centered on a first predetermined frequency from one of the sensors and adapted to CMOS logic.

14. Device according to claim 13, characterized in that the impact counter (CLKi) is activated by a stable clock, counting being authorized as long as the envelope of successive echoes of the wave train generated in the plate at neighborhood of the first predetermined frequency remains above a determined threshold.

15. Device according to one of claims 12 to 14, characterized in that means are also provided for determining the intensity of a signal transmitted by the source to the plate around at least one different centering frequency of the first predetermined frequency.

16. Device according to claim 15, characterized in that there is also provided at least one other counter (BF) associated individually with a centering frequency and capable of counting the number of echoes on the edges of the plate of a acoustic wave generated around said associated centering frequency by an interaction of the source and the plate.

17. Device according to claim 15 or 16, characterized in that means are provided for determining the nature of the source as a function of the intensity of the signal transmitted from the source to the plate around the first predetermined frequency and the centering frequency (s).

18. Device according to one of claims 11 to 17, characterized in that means are provided for sending a user a light message via a peripheral connected to the processing means, indicating to him if an interaction has generated an acoustic signal within a desired reverb range.

19. Device according to one of claims 11 to 18, characterized in that according to a threshold value reached by one or more counters each associated individually with a frequency, the device is capable of triggering preventive actions for the safety of the plate, for example by activating an alarm or by triggering the closure of a metal curtain covering the plate.

20 Device according to one of the preceding claims, characterized in that the source is able to excite the plate without said plate transmitting sound waves to the ambient environment.

21. Device according to claim 20, characterized in that the source is a stylus (100) of which a hollow longitudinal main body (101) comprises a source of electrical energy (102), a generator of electrical pulses (103) capable to be powered by the electric power source, a ceramic piezoelectric disc (104) capable of being excited by the pulse generator to bring into longitudinal resonance a cavity (105) of the main body filled with a component having a mechanical impedance comparable to that of the material constituting the main body, said cavity of the main body being closed by a pellet (107) made of a soft material of comparable impedance such as silicone.

22. Device according to claim 21, characterized in that the electric pulse generator is capable of continuously emitting bursts of pulses around 100 kHz and the hollow main body is made of a material entering into resonance around this frequency , for example a plastic material.

23. Device according to claim 21 or 22, characterized in that the component filling said cavity of the main body is an antifreeze liquid (106).

24. Device according to one of claims 21 to 23, characterized in that the stylus comprises a power switch to control the emission of acoustic waves.

25. Device according to one of claims 1 to 19, characterized in that the plate is capable of being excited by a compression wave generated by the voice of a user, and the output of the broadband preamplification stage ( BFX-, BFX +) of at least one of the electronic circuits associated with the respective sensors is derived towards low-pass amplifiers, for example Bessel filters of order 6 limited to the audible band.

26. Device according to claim 25, characterized in that the output of said low-pass amplifiers is diverted to an output connector of the stereo Jack type and subsequently to an audio acquisition card.

27. Device according to claim 25 or 26, characterized in that one or more electronic band-cut filters are inserted afterwards low-pass filters to compensate for the plate's own resonances at certain frequencies.

28. Device according to one of claims 25 to 27, characterized in that it comprises a software filter with reverse transfer function of the acousto-electric frequency response of the plate on a given frequency band of a few kiloHertz, for processing audio signals from the glass plate.

29. Device according to one of the preceding claims, characterized in that one of the sensors is capable of being switched into a transmitter of a packet of ultrasonic waves in order to trigger a plate integrity test comprising the measurement:

• propagation time differences (t _{y + y} ., T _{y + x-} -t _{y + x +} ) of the wave packet between the different sensors,

• and the reverberation of the wave packet using an impact counter, and the device comprises means for comparing said reverberation value with reference values and means for triggering preventive actions for the safety of the plate, for example by activating an alarm or closing a metal curtain covering the plate.

30. Device according to claim 29, characterized in that the integrity test is triggered by a write instruction at a memory address, starting a general counter and an oscillator (R130, R129, C159) from which a logic burst (SOURC) and its logic inverse (SINK) are capable of actuating the grids of electronic components controlling the adaptation of the logic burst to a higher voltage applied to the sensor switched to the transmitter, said higher voltage being obtained at using the oscillator and a diode pump connected to reservoir capacitors (C126, C131).

31. Device according to claim 30, characterized in that the oscillator is a relaxation oscillator operating at approximately 100 kHz, said higher voltage is of the order of 60 Volts and said electronic components are NMOS transistors (T7, T13 , T8, T14) and PMOS (T12.T19).

32. Device according to claim 30 or 31, characterized in that the input stage (AOPY +) of the electronic circuit associated with the sensor switched to the transmitter is protected from the higher voltage burst by NMOS switching transistors (T11, T18) blocked during the burst and passers-by when the sensor able to be switched to transmitter is in reception mode.

33. Device according to one of claims 29 to 32, characterized in that the integrity test of the plate is done at the start of the device and just after an interaction between the source and the plate.

34. Device according to one of claims 29 to 33 characterized in that additional pairs of transducers are dedicated to the integrity test.

35. Device according to claims 2 and 34 taken in combination, characterized in that the pairs of additional transducers are also four in number and define a second rhombus oriented at 45 ° relative to the rhombus of the sensors, the pairs of additional transducers participating also to the measurement of coordinates of interaction points between the source and the plate.

36. Device according to one of the preceding claims constituting a peripheral interface with a machine which receives the signals from the processing means, and which according to said signals controls the execution of files contained in the computer by various peripherals of known types connected to the computer such as for example in a nonlimiting manner a video projector, sources lights, speakers, a printer, or even an automation unit controlling a mechanical action such as closing a protective curtain.

37. Device according to claim 36, characterized in that said machine is a computer provided with a screen.

38. Device according to claim 37, characterized in that a pixel of coordinates (N _cx , N _cy ) on the computer screen showing an image of the plate is associated with a physical point of the coordinate plate ( x _r , y _r ), the conversion of the coordinates comprising the following operations: a) visual marking of the center (O) of the sensors, b) recording of a digital image including the origin (O), c) delimitation on the digital image of a working frame of center C, and calculation of the length (N _max χ) and the height (Nm _ax γ) in pixels of the working frame, d) definition of a homothetic correspondence which gives the working frame scale, i.e. the length L and the height I in millimeters corresponding on the plate to the drawn working frame, e) positioning on the digital image of a cursor pointing at the center of the sensors, c '' is to say on the origin of the real coordinates (x _r , y _r ) of the plate, visually identified in a ) so that any pixel (q) of any N _xq , N _yq coordinates in the working frame will be linked to its real coordinates (x _r , y _r ) according to the formula:

N f = ^X '^<N t ^m> + jx

. . H nwx c. ,

N> .. v. = t _ - - + N,.,.

39. Device according to claim 38 characterized in that a portion of the plate defined as a working frame equivalent to the computer screen is divisible into zones of any shape, each zone corresponding to a domain of the digital image of the plate which is stored in the computer and which is able to be activated by an interaction at a place (x _r , y _. ) of the plate belonging to the zone, to control the execution of different groups of programs of l 'computer.

40. Device according to one of claims 37 to 39 characterized in that the computer is able to carry out at least one of the following operations:

• a time delay which neutralizes the detection of the acoustic waves for a given time interval after the detection of an interaction of the source and the plate,

• tests of the proper functioning of a signal acquisition card, by visualizing on the computer screen the location of a physical impact on the plate as well as the value of impact counters,

• a global backup of the computer files linked to the operation of the plate (in particular the files constituting the sequences associated with each of the fields of the digital image of the plate) according to the following rules: a) all the files forming a sequence are saved in a directory bearing the name of the domain with which this sequence is associated; b) all the domain directories are in a single directory containing in addition the source file,

• a statistical module to exploit the information collected by the computer, and indicating in particular the number of interactions that have been carried out during a given period with each zone of the plate's working environment.

41. Device according to one of the preceding claims, characterized by complementary means allowing the lighting, for example at night, of the lighting of the plate or of an area of the plate following an impact on any or predetermined part of the frame. plate work.

42. Device according to one of the preceding claims, characterized in that the transducers constituting each sensor (SMC1, SMC2, SMC3, SMC4) are connected to a resistance bridge associated with the sensor before attacking the inputs (AOPX-, AOPX +, AOPY-, AOPY +) wideband preamplifiers of the electronic circuits associated with the sensor.

43. Device according to one of the preceding claims, characterized in that the plate is made of glass.