Method and system for determining the operating frequency of an acoustic transmission system

Abstract

Method for determining an operating frequency of an acoustic transmission system (ATS) comprising a primary element (PE) and a secondary element (SE) arranged on either side of a wall, the primary element comprising a first electro-acoustic transducer (T1) for generating and detecting acoustic waves (AO), the secondary element (SE) comprising a second electro-acoustic transducer (T2) and being configured to modify in a controlled manner an electrical impedance across said second transducer between an open circuit condition and a short circuit condition;characterized in that said operating frequency is determined from the frequency-dependent evolution of the phase difference between an input impedance, on the side of the primary element (PE), of an electrical quadrupole modeling the assembly formed by the first (T1) and second (T2) acoustically coupled transducers, with the second transducer (T2) in short circuit and open circuit. System for implementing such a method. Figure for the abstract: Fig. 2;

Description

Title of the invention: Method and system for determining the operating frequency of an acoustic transmission system

[0001] The invention relates to the field of acoustic power and data transmission systems (TAPD, or APDT).

[0002] Such systems enable communication—generally over short distances, i.e., on the order of centimeters or at most meters—between equipment located on either side of a wall, particularly a metal one. The communicating equipment may be, for example, one or more sensors on the one hand and a control station for said sensors on the other.

[0003] The use of radio frequency or microwave communication techniques—for example, RFID, Bluetooth, WiFi—is not possible because the metallic wall absorbs electromagnetic waves. For the same reasons, it is not possible to use electromagnetic induction to power the sensors. Furthermore, the use of wired techniques would require drilling openings in the wall, which could pose problems related to cost, structural integrity, and sealing.

[0004] Under these conditions, it is known to use acoustic waves to transmit power and / or data through the wall.

[0005] For example, a primary element (control station) located on an accessible side of a wall, such as the outside of a pipe, can emit acoustic waves toward a secondary element (sensor) located on an inaccessible side of said wall, such as the inside of the pipe. On the sensor side, an electro-acoustic transducer, typically piezoelectric, converts the energy of the acoustic waves into an electric current that can be used to directly power electronic circuits and / or to charge a battery or a supercapacitor. Furthermore, the acoustic waves emitted by the primary element can be modulated to transmit data to the secondary element, for example, sensor configuration or interrogation data.The sensor can also transmit data, such as measurements, to the control station either by emitting acoustic waves itself or by modulating the phase of the acoustic waves originating from the control station and reflected back to it. In the latter case, this phase modulation can be achieved simply by changing the impedance across the electro-acoustic transducer; this is known as impedance switching or impedance modulation communication. See, for example, (Takahashi 2019).

[0006] Power transmission varies significantly with the frequency of the acoustic waves due to resonance effects in the wall. This is illustrated in [Fig. 1], which shows the variation of the energy transmission efficiency 7 (in percent), the maximum normalized transmitted power (P9), and the maximum normalized voltage V? generated by the transducer during reception as a function of frequency. The power is normalized with respect to the square of the input voltage, and the output voltage is normalized with respect to the input voltage Vp. For each frequency, an optimal impedance value was considered across the transducer terminals.

[0007] However, these resonance frequencies vary depending on the temperature, changes in geometry, etc. The operating frequency of the system cannot therefore be determined once and for all, but must be able to be determined regularly, and as simply as possible.

[0008] Document EP 4 080 791 discloses a method for determining the frequency that maximizes the power transfer from the primary element to the secondary element. According to this method, three frequency sweeps of the acoustic waves are performed for three different values ​​of an impedance across the terminals of the electro-acoustic transducer of the secondary element. This makes it possible to evaluate the impedance matrix of the quadrupole formed by the wall and the two transducers, and to deduce the desired frequency.

[0009] Document EP 3 671 872 proposes choosing the frequency at which the phase of the input admittance is minimal, with the secondary element transducer in open circuit. This does not guarantee maximum efficiency or transmitted power, nor does it allow for the determination of the electrical load to be connected.

[0010] Document EP 2 832 016 proposes selecting the frequency that maximizes the voltage across a storage capacitor in the secondary element. This requires prior establishment of communication so that the secondary element can transmit charge level information from the capacitor to the primary element.

[0011] Document US2010 / 0027379 describes an operation in which the operating frequency is determined by maximizing the amplitude of the received signal, which also requires the prior establishment of communication between the secondary element and the primary element.

[0012] The methods known in the prior art are not entirely satisfactory. Indeed, the present inventors have realized that they do not, in all cases, guarantee optimal operation of the transmission system, that is to say, operation guaranteeing both: - a sufficiently high power level from the primary element to the secondary; - a high efficiency - defined as the ratio between the power received by the secondary element and the power consumed by the primary element; - a sufficiently high voltage across the transducer of the secondary element, particularly compared to the threshold voltages of diodes connected to said transducer; and - a high communication speed.

[0013] Furthermore, some of the prior art methods are complex to implement and require the prior establishment of communication between the secondary and primary elements. This can be problematic when the energy storage device (battery or supercapacitor) of the secondary element is completely discharged; it is then necessary to carry out a preliminary charging step of this storage device, but "blindly," without knowing the optimal operating frequency.

[0014] The invention aims to overcome, in whole or in part, these limitations of the prior art.

[0015] In accordance with the invention, this objective is achieved by determining the operating frequency as a function of the difference Aq> between the phase of the input impedance of an electrical quadrupole modeling the assembly formed by the transducers of the primary and secondary elements acoustically coupled when the transducer of the secondary element is short-circuited, and the phase of said input impedance when the transducer of the secondary element is open-circuited.

[0016] More specifically, the operating frequency can be the frequency that maximizes Aq>. This frequency is optimal for impedance modulation communication and allows for the highest data rate. Furthermore, the present inventors have found that it is nearly optimal in terms of transmitted power and efficiency.

[0017] Alternatively, a higher frequency can be chosen, but one lower than the value for which Aq>=0. Such a choice improves the transmitted power and output voltage, but at the cost of a degraded data rate. The optimal compromise will depend on the specific application considered, but generally, the communication data rate will be prioritized by choosing an operating frequency that ensures a phase difference Aq> at least equal to half its maximum value.

[0018] An object of the invention is therefore a method for determining the operating frequency of an acoustic transmission system comprising a primary element and a secondary element, the primary element comprising a first electro-acoustic transducer configured to generate acoustic waves of variable frequency in the direction of the secondary element through a wall and for detecting reflections of said acoustic waves by the secondary element, the secondary element comprising a second electro-acoustic transducer adapted to convert said acoustic waves into an electrical supply current, said secondary element being further configured to modify in a controlled manner an electrical impedance across said second transducer between an open-circuit condition and a short-circuit condition; characterized in that it comprises the following steps: a) emission of said acoustic waves from the primary element to the secondary element, while keeping the second transducer in short circuit; b) simultaneously with step a), measurement of a first phase of the input impedance, on the side of the primary element, of an electrical quadrupole modeling the set formed by the first and second acoustically coupled transducers, with the second transducer short-circuited; c) emission of said acoustic waves from the primary element to the secondary element, while keeping the second transducer in an open circuit; d) simultaneously with step c), measurement of a second phase of the input impedance, on the side of the primary element, of an electrical quadrupole modeling the assembly formed by the first and second acoustically coupled transducers, with the second transducer in open circuit; e) calculation of the difference Aq> between the first and second phases; said steps a) to e) being repeated for a plurality of frequencies covering an operating range of the acoustic transmission system; and then f) determination of said operating frequency as a function of said difference between the first and second phases calculated for said plurality of frequencies.

[0019] According to particular embodiments of this process:

[0020] The operating frequency determined in step f) can be between a first value fi maximizing said difference between the first and second phase and a second value f2 canceling it, the latter value being excluded.

[0021] More particularly, the operating frequency determined during step f) can be between said first value maximizing said difference between the first and second phase and a third value f3, between said first and second values ​​and such that said difference between the first and second phase takes a value not less than half of its maximum value.

[0022] The method may include, in the secondary element, the actuation of a switching device to switch from a condition in which the second transducer is short-circuited to a condition in which the second transducer is open-circuited and vice versa.

[0023] Alternatively, the secondary element may comprise a pair of diodes mounted back-to-back in parallel with said second transducer, the method comprising the emission, by said primary element, at successive instants, of said acoustic waves at a first intensity level sufficiently low so that a voltage generated by said second transducer excited by said acoustic waves is less than a single voltage of said diodes, and at a second intensity level sufficiently high so that a voltage generated by said second transducer excited by said acoustic waves is greater than a single voltage of said diodes.

[0024] Steps a) to e) can be implemented successively for each frequency of said plurality of frequencies.

[0025] Alternatively, steps a) and b) may first be implemented for said plurality of frequencies, and then steps c) and d) may in turn be implemented for the same frequencies, or vice versa.

[0026] Another object of the invention is an acoustic transmission system comprising a primary element and a secondary element, the primary element comprising a first electro-acoustic transducer configured to generate acoustic waves of variable frequency in the direction of the secondary element through a wall and to detect reflections of said acoustic waves by the secondary element, the secondary element comprising a second electro-acoustic transducer adapted to convert said acoustic waves into an electrical supply current, said secondary element being further configured to modify in a controlled manner an electrical impedance across said second transducer between an open circuit condition and a short circuit condition;characterized in that the transmission system includes a first control means configured to drive the primary element so as to, for a plurality of frequencies sweeping an operating range of the acoustic transmission system: ; a) emit said acoustic waves from the primary element to the secondary element, while keeping the second transducer in short circuit; b) simultaneously with step a), measure a first phase of the input impedance, on the side of the primary element, of an electrical quadrupole modeling the set formed by the first and second acoustically coupled transducers, with the second transducer short-circuited; c) emit said acoustic waves from the primary element to the secondary element, while keeping the second transducer in an open circuit; d) simultaneously with step c), measure a second phase of the input impedance, on the side of the primary element, of an electrical quadrupole modeling the assembly formed by the first and second acoustically coupled transducers, with the second transducer in open circuit; the control method is also configured for e) calculate the difference Aq> between the first and second phases for each frequency of said plurality of frequencies; and for f) determine said operating frequency as a function of said difference between the first and second phases calculated for said plurality of frequencies.

[0027] According to particular embodiments of this system:

[0028] Said control means can be configured to determine said operating frequency between a first value fi maximizing said difference between the first and second phase and a second value f2 canceling it, the latter value being excluded.

[0029] More particularly, said control means can be configured to determine said operating frequency between said first value maximizing said difference between the first and second phase and a third value f3, between said first and second values ​​and such that said difference between the first and second phase takes a value not less than half of its maximum value.

[0030] The system may include, in the secondary element, a second control means and a switching device, said second control means being configured to drive the actuation of said switching device to move it from a condition in which the second transducer is short-circuited to a condition in which the second transducer is open-circuited and vice versa.

[0031] Alternatively, the secondary element may include a pair of diodes mounted back-to-back in parallel with said second transducer, said first control means also being configured to drive the first transducer so that it emits, at successive instants, said acoustic waves at a first intensity level sufficiently low so that a voltage generated by said second transducer excited by said acoustic waves is less than a single voltage of said diodes, and at a second intensity level sufficiently high so that a voltage generated by said second transducer excited by said acoustic waves is greater than a single voltage of said diodes.

[0032] Said first control means, or said first and second control means, can be configured to implement steps a) to e) successively for each frequency of said plurality of frequencies.

[0033] Alternatively, said first control means, or said first and second control means, may be configured to implement first steps a) and b) for said plurality of frequencies, then steps c) and d) for the same frequencies, or vice versa.

[0034] Other features, details and advantages of the invention will become apparent from the description given with reference to the accompanying drawings provided by way of example, which represent, respectively:

[0035] [Fig. 1], already mentioned above, a graph of the variation of the energy transmission efficiency, the maximum power transmitted and the maximum voltage generated by a transducer in reception as a function of the frequency of an acoustic wave transmitted through a wall;

[0036] [Fig.2], the schematic diagram of an acoustic power and data transmission system suitable for implementing the invention;

[0037] [Fig.3], graphics illustrating the principle underlying the invention;

[0038] [Fig.4], a flowchart of a method for determining an operating frequency according to an embodiment of the invention;

[0039] [Fig.5], a flowchart of a method for determining an operating frequency according to another embodiment of the invention;

[0040] [Fig.6], the schematic diagram of a primary element of an acoustic power and data transmission system according to an embodiment of the invention;

[0041] [Fig.7], the schematic diagram of a secondary element of an acoustic power and data transmission system according to one embodiment of the invention; and

[0042] [Fig.8], the schematic diagram of a primary element of an acoustic power and data transmission system according to another embodiment of the invention.

[0043] The acoustic power and data transmission system SA of [Fig. 2] comprises a primary element EP and a secondary element ES arranged on either side of a wall P, for example metallic. The primary element EP comprises a first electro-acoustic transducer (typically piezoelectric) T1 and a first control means - consisting of one or more electronic circuits - MCP electrically connected to the first electro-acoustic transducer TL. Similarly, the secondary element ES comprises a second electro-acoustic transducer (typically piezoelectric) T2 and a second control means - consisting of one or more electronic circuits - MCS electrically connected to the second electro-acoustic transducer T2.The first electro-acoustic transducer T1 is used for both transmission and reception: it is supplied with alternating electric current by the first control means MCP to generate acoustic waves OA which propagate through the wall P. The acoustic waves OA are partially reflected and partially absorbed by the second transducer T2.

[0044] The acoustic power absorbed by the second transducer T2 is converted into electrical power which is used directly to power the second means of MCS control and / or to charge a battery or supercapacitor (not shown) power supply.

[0045] The reflected fraction of the acoustic waves OA passes back through the wall P and influences the voltage Vi across the transducer Tl and / or the current h flowing through it. This voltage and current are read by the first control means MCP.

[0046] The (complex) acoustic wave reflection coefficient of the second transducer T2 depends, among other things, on the electrical impedance connected to its terminals, the value of which can be varied by the second control means MCS. Therefore, the first control means MCP, by analyzing the phase and amplitude of the voltage Vi and the current Ib, can detect a change in said impedance. This effect is exploited to transmit information from the secondary element ES to the primary element EP. Typically, this transmission occurs in a binary fashion, with the impedance across the terminals of the second transducer T2 switching between a high value, in principle infinite—in which case the transducer can be considered open-circuited—and a low value, in principle zero, in which case the transducer can be considered short-circuited.Alternatively, it is possible to switch between a "high" but not infinite impedance and a "low" impedance that does not constitute a short circuit. This allows the secondary element to communicate while harvesting energy from the acoustic waves. It is also possible to use more than two impedance values.

[0047] The assembly consisting of the two transducers T1, T2 acoustically coupled through the wall P can be modeled by an electrical four-terminal network. Let Vi be the voltage across the transducer Ti and h the current entering said transducer; similarly, let V2 be the voltage across the transducer T2 and I2 the current entering said transducer transducer. So we can write:

[0048] [VA / Zn Zl2\[IA

[0049] Where Zn, Z[2=Z2i and Z22 are complex values ​​having the dimensions of a impedance.

[0050] When the transducer T2 is in open circuit, I2=0; therefore y! = V™ - and y? = y™ — Z^I^ (where the exponent "co" means "open circuit") "). We can therefore deduce:

[0051]

[0052]

[0053] When the transducer T2 is short-circuited, y? = y^ = 0; therefore

[0054] yj = y^c = Z ( ( 1 - c2 ), and consequently

[0055]

[0056] where the superscript "ce" means "short circuit" and the coupling parameter c is given by

[0057] .

[0058] The input impedance 7 _ X is defined, which can be determined exclusively From measurements taken by the first MCP control method, we have, when T2 is open-circuited, Z^ = Zj, and when T2 is short-circuited, Z^ = Zj f ( 1 - c1 ). The variation of the input impedance of the two-port network when T2 changes from its open-circuit to its short-circuited state is therefore... I z[2 I en A î ^in l ~ I ^in I ~ ! ^in I — | Z22 | module and A^ = arg ( Z^ ) - arg ( Z™ ) in phase.

[0059] In practice, to extract the information encoded in the impedance switching at the terminals of the transducer T2, the first control means preferably detects the phase of the input impedance variation Aç>, the measurement of which is less sensitive to noise than that of the module A j|, the higher the signal-to-noise ratio of the communication channel from ES to EP, the higher the transmission rate can be.

[0060] These different impedances, and therefore also Aç>, are generally a function of the frequency / excitation of the transducer Tl (and therefore of the acoustic waves OA).

[0061] Figure 3 shows how arg(Z^), arg(^), a, &p = (top panel), the power transmission efficiency Φ (2nd panel from top), the maximum normalized transmitted power P2 (3rd panel from top), and the maximum normalized voltage generated by the receiving transducer (bottom panel) vary with frequency in an acoustic power and data transmission system (AS). The curves in Figure 3 vary to some extent depending on the implementation details of the AS system, but their shape remains remarkably constant. The conclusions that can be drawn from them are therefore of general scope. It is noted that the frequency Φ that maximizes A2 allows for an efficiency Φ and a transmitted power P2 close to their respective maxima. This is therefore an interesting operating point for the AS system.When the operating frequency is increased beyond f, A^p decreases—first slowly, then more rapidly—while P2 increases slightly and f2 increases more significantly. For f=f2, 7 / ct V~> are maximized, but communication is impossible because A^> = 0. Advantageously, therefore, an operating frequency equal to f (to maximize the transmission rate) or between f and f2 will be chosen.

[0062]

[0063] (this last value being excluded) and preferably between / 7 and / 3, such that } =4 Afp ( f (To find a compromise between the maximization of A(p and that of P2 and V2). Figure 4 shows a flowchart of a method for determining an operating frequency of the SA system according to an embodiment of the invention. First, a plurality of discrete frequencies / 1' sweeping an operating band F is chosen. The index i is first initialized: i=l.

[0064] Next - step a) - the first control means MCP drives the first transducer T1 to emit acoustic waves at the frequency / '7' while the second transducer T2 is kept short-circuited.

[0065] Meanwhile – step b), which is in fact simultaneous with step a) – the first MCP control means measures (y / _ arg [ ' or P'us exactly calculates ^cc y*) ) from measurements of Vi and L

[0066] Then - step c) - the first control means MCP drives the first transducer T1 to emit acoustic waves at the frequency / '7' while the second transducer T2 is kept in open circuit.

[0067] Meanwhile - step d), which is in fact simultaneous with step c) - the first MCP control means measures ( y Ü j _ arg ( y ) ] 'or P^us exactly calculates from measurements of V, and I,

[0068] Next - step e) - the first MCP control means calculates A^ / ') =^(0- ^( / °)•

[0069] At this point, the index i is incremented and steps a) - e) are iterated until said index reaches its maximum value imax.

[0070] In the last step, the operating frequency f / onc is determined from for all values ​​of i between 1 and imax. As As explained above, we can preferentially choose "[ r / ■ V J fonc [y 1' 2 / G [ / / 1, or / - f J Jonc LJ 3 j J jonc J 1

[0071] It is understood that it is possible to reverse the order of steps a / b) and c / d). Furthermore, it is possible to store the values ​​of qycc (Oet ^co ) to perform step e) of calculation of a after iterating over all admissible values ​​of i.

[0072] It is also possible that steps b) and d) provide the complex values ​​of Zin and that, in step e), the complex value AZ is calculated, and then its argument is extracted to obtain Ap.

[0073] Figure 5 shows another flowchart of a method for determining an operating frequency of the SA system according to an alternative embodiment of the invention. In this embodiment, steps a) and b) are first implemented iteratively for all permissible values ​​of i, and then steps c) and d) are iterated. Steps e) and f) are implemented after the two sets of iterations. As in the previous case, it is possible to reverse the order of steps a / b) and c / d). For the implementation of this embodiment, signals exhibiting continuous, typically linear, frequency modulation (a "chirp") can be used to drive the TL transducer.

[0074] Figure 6 shows a functional diagram of a primary element EP of a system SA according to an embodiment of the invention.

[0075] The primary element EP mainly comprises the MCP control means and the MCP transducer. Other components will generally be present: a power supply, an input-output interface, etc. These components are generally conventional; in the following, we will focus on the structure and operation of the MCP control means.

[0076] Inside the first control means MCP, a digital processor PROC controls a driver circuit CPT, configured to generate an excitation signal for the first transducer Tl, to which it is electrically connected. The processor PROC determines the frequency of said excitation signal and, if applicable, its amplitude (the importance of the amplitude will be discussed later, with reference to [Fig. 8]). A first differential amplifier ADV reads the voltage Vi across the transducer TL. Its output is digitized by a first analog-to-digital converter CANV. A second differential amplifier ADI, having two inputs connected to the terminals of a shunt resistor R, arranged in series with the conductor connecting the driver circuit CPT to the transducer Tl, measures the current L at the input of said transducer. Its output is digitized by a second analog-to-digital converter CANI.The digital signals output from the CANV, CANI converters are supplied as input to the PROC processor.

[0077] The PROC processor can be a specially programmed microprocessor or a dedicated digital circuit. In any case, it is configured to, in a first phase: - control the CPT control circuit to perform an operating sweep, so that the transducer excitation signal Tl successively takes the values ​​mentioned above; - acquire CANV, CANI converters of digital signals of V / ^), Ii( / W) for each of these frequencies;

[0078]

[0079]

[0080]

[0081] - determine the operating frequency f jonc as explained above. And in a second phase: - control the CPT control circuit to emit an acoustic wave OA at the frequency ffonc; - acquire CANV, CANI converters of digital signals of Vi( / Ii( ); rush7 - determine the phase variations of p and deduce a sequence of bits transmitted, by impedance switching, by the secondary element. Figure 7 shows a partial functional diagram of a secondary element of an SA system according to one embodiment of the invention. In this embodiment, the secondary element ES includes a sensor C that acquires data from the external environment (for example, temperature) and converts it into an electrical signal that is supplied as input to the second control means MCS. MCS drives a DC switching device (in the figure, a MOS transistor) connected to the terminals of the second transducer T2. The information from sensor C is transmitted to the primary element using impedance switching: when the DC transistor is conducting, transducer T2 is short-circuited; when it is blocking, it is open-circuited. The switching of the DC transistor is also used to periodically implement a method for determining the operating frequency as described above. In this embodiment, the secondary element plays an active role.It must therefore have been previously powered by the acoustic waves emitted by the primary element, and be able to receive commands from the latter. The means of energy recovery and storage (rectifier bridge, battery or supercapacitor...) and those of signal reception (which can be integrated into the second MCS control means) are not shown in the figure because they are conventional. Figure 8 illustrates the operating principle of a secondary element that plays a purely passive role in the implementation of the operating frequency determination method. Therefore, the operating frequency can be determined even if the secondary element is inactive or completely discharged. In this embodiment, two diodes D1, D2 arranged back-to-back are connected to the terminals of the transducer T2. When the acoustic waves OA are sufficiently intense, the voltage generated by T2 exceeds the threshold voltage of the diodes, which then behave as short circuits. Conversely, when the intensity of the acoustic waves decreases, the voltage generated by T2 is lower than the threshold voltage of the diodes, which then behave as open circuits. The impedance switching on the secondary side is therefore driven by the primary element via the control of the amplitude of the acoustic waves generated by T1.

[0082] Even in the embodiment of [Fig.8] the secondary element generally includes means for energy recovery and storage (rectifier bridge, battery or supercapacitor...) means for receiving signals and active means for impedance switching (DC transistor), but which are not shown in the figure because they play no role in the process according to the invention.

[0083] Bibliography

[0084] (Takahashi 2019): VL Takahashi et al. “Ultrasonic Power and Data Transfer through Multiple Curved Layers Applied to Pipe Instrumentation” Sensors 2019, 19, 4074

Claims

1. Demands Method for determining an operating frequency of an acoustic transmission system (ATS) comprising a primary element (PE) and a secondary element (SE), the primary element comprising a first electro-acoustic transducer (TA1) configured to generate acoustic waves (AO) of variable frequency in the direction of the secondary element through a wall (P) and to detect reflections of said acoustic waves by the secondary element, the secondary element (SE) comprising a second electro-acoustic transducer (T2) adapted to convert said acoustic waves into an electrical supply current, said secondary element being further configured to modify in a controlled manner an electrical impedance across said second transducer between an open circuit condition and a short circuit condition;characterized in that it comprises the following steps: a) emission of said acoustic waves (OA) from the primary element (EP) to the secondary element (ES), while keeping the second transducer (T2) in short circuit; b) simultaneously with step a), measurement of a first phase of the input impedance, on the side of the primary element (EP), of an electrical quadrupole modeling the set formed by the first (T1) and the second (T2) acoustically coupled transducer, with the second transducer (T2) in short circuit; c) emission of said acoustic waves (OA) from the primary element (EP) to the secondary element (ES), while keeping the second transducer (T2) in open circuit; d) simultaneously with step c), measurement of a second phase of the input impedance, on the side of the primary element (EP), of an electrical quadrupole modeling the set formed by the first and second acoustically coupled transducers, with the second transducer (T2) in open circuit; e) calculation of the difference (Aq>) between the first and second phases; said steps a) to e) being repeated for a plurality of frequencies sweeping an operating range of the acoustic transmission system; and then f) determination of said operating frequency as a function of said difference (Aq>) between the first and second phases calculated for said plurality of frequencies.

2. A method according to claim 1 wherein the operating frequency determined in step f) is between a first value (fj) maximizing said difference between the first and second phases and a second value (f2) canceling it, the latter value being excluded.

3. A method according to claim 2 wherein the operating frequency determined in step f) is between said first value (fi) maximizing said difference between the first and second phases and a third value (f 5), between said first (fj) and second (f2) values ​​and such that said difference between the first and second phases takes a value not less than half of its maximum value.

4. A method according to any one of the preceding claims comprising, in the secondary element (ES), the actuation of a switching device (DC) to go from a condition in which the second transducer (T2) is short-circuited to a condition in which the second transducer (T2) is open-circuited and vice versa.

5. A method according to any one of claims 1 to 3 wherein the secondary element comprises a pair of diodes (D1, D2) mounted back-to-back in parallel with said second transducer, the method comprising the emission, by said primary element, at successive instants, of said acoustic waves (OA) at a first intensity level sufficiently low so that a voltage (V2) generated by said second transducer (T2) excited by said acoustic waves (OA) is less than a single voltage of said diodes (D1, D2), and at a second intensity level sufficiently high so that a voltage (V2) generated by said second transducer (T2) excited by said acoustic waves is greater than a single voltage of said diodes.

6. A method according to any one of the preceding claims wherein steps a) to e) are carried out successively for each frequency of said plurality of frequencies.

7. A method according to any one of claims 1 to 5 wherein steps a) and b) are first carried out for said plurality of frequencies, and then steps c) and d) are in turn carried out for the same frequencies, or vice versa.

8. Acoustic transmission system comprising a primary element (EP) and a secondary element (ES), the primary element (EP) comprising a first electro-acoustic transducer (T1) configured to generate acoustic waves (OA) of variable frequency in the direction of the secondary element (ES) through a wall (P) and to detect reflections of said acoustic waves by the secondary element (ES), the secondary element (ES) comprising a second electro-acoustic transducer (T2) adapted to convert said acoustic waves into an electrical supply current, said secondary element being further configured to modify in a controlled manner an electrical impedance across said second transducer (T2) between an open circuit condition and a short circuit condition;characterized in that the transmission system comprises a first control means (MCP) configured to drive the primary element so as to, for a plurality of frequencies sweeping an operating range of the acoustic transmission system: a) emit said acoustic waves from the primary element to the secondary element, while keeping the second transducer short-circuited; b) simultaneously with step a), measure a first phase of the input impedance, on the side of the primary element, of an electrical quadrupole modeling the assembly formed by the first and second acoustically coupled transducers, with the second transducer short-circuited; c) emit said acoustic waves from the primary element to the secondary element, while keeping the second transducer open-circuited;d) simultaneously with step c), measure a second phase of the input impedance, on the side of the primary element, of an electrical quadrupole modeling the assembly formed by the first and second acoustically coupled transducers, with the second transducer in open circuit; the control means being also configured to; e) calculate the difference between the first and second phases for each frequency of said plurality of frequencies; and for f) determine said operating frequency as a function of said difference between the first and second phases calculated for said plurality of frequencies.

9. System according to claim 8 wherein said control means (MCP) is configured to determine said operating frequency between a first value (fj) maximizing said difference between the first and second phase and a second value (f2) canceling it, the latter value being excluded.

10. System according to claim 8 wherein said control means (MCP) is configured to determine said operating frequency between said first value (fi) maximizing said difference between the first and second phase and a third value (fj) between said first (fj) and second (f2) values ​​and such that said difference between the first and second phase takes a value not less than half of its maximum value.

11. System according to any one of claims 8 to 10 comprising, in the secondary element (ES), a second control means (MCS) and a switching device (DC), said second control means being configured to drive the actuation of said switching device to move it from a condition in which the second transducer (T2) is short-circuited to a condition in which the second transducer is open-circuited and vice versa (T2).

12. A system according to any one of claims 8 to 10, wherein the secondary element comprises a pair of diodes (D1, D2) mounted back-to-back in parallel with said second transducer (T2), said first control means (MCP) also being configured to drive the first transducer (T1) so as to emit, at successive instants, said acoustic waves (AO) at a first intensity level sufficiently low that a voltage (V2) generated by said second transducer (T2) excited by said acoustic waves is less than a single voltage of said diodes (D1, D2), and at a second intensity level sufficiently high that a voltage (V2) generated by said second transducer (T 12) excited by said acoustic waves is greater than a single voltage of said diodes (D1, D2).

13. System according to any one of claims 8 to 12 wherein said first control means, or said first and second control means, are configured to implement steps a) to e) successively for each frequency of said plurality of frequencies.

14. System according to any one of claims 8 to 12 wherein said first control means, or said first and second control means, are configured to implement first steps a) and b) for said plurality of frequencies, then steps c) and d) for the same frequencies, or vice versa.

Images