Method for acoustically locating an object

Abstract

Method for the acoustic localization of an object. Method for localizing an object (O), comprising the steps in which an object equipped with at least one acoustic emitter (12) is provided; at least first and second acoustic receivers (14) are provided; an acoustic signal (x(t)) is emitted; upon receipt of an acoustic signal (y1(t),y2(t),y3(t)) by an acoustic receiver, a cross-correlation is performed between the received acoustic signal and a reference signal; a transformation is applied to said determined correlation signal, so as to determine a transformed correlation signal (Sxy1 f(t),Sxy2 f(t),Sxy3f(t)) whose shape follows the envelope of the correlation signal; the set of local maxima (M1,M2) of the transformed correlation signal (Sxy1f(t),Sxy2f(t),Sxy3f(t)) prior to a predetermined final instant (tf) is determined;An arrival time (t01, t02, t03) of the received acoustic signal is determined, based on the first local maximum (M1) of the transformed correlation signal considered within the predetermined time window; the difference(s) in arrival times (TDOA1-2, TDOA2-3, TDOA1-3) are calculated; position data (Gx, Gy, Gz) of the object are determined. Figure for the abbreviation: Fig. 1.

Description

Title of the invention: Method for acoustically locating an object Technical field

[0001] The present invention relates to the technical field of methods for locating an object making it possible to determine position data of an object in order to locate the latter. Such localization methods are notably implemented to guide a robot, for example a charging robot making it possible to electrically recharge an electric storage battery of an electric motor vehicle.

[0002] The invention relates more specifically to the technical field of location methods using acoustic techniques implementing one or more acoustic transmitter(s) and one or more acoustic receiver(s). Prior art

[0003] Acoustic localization methods are known in which signals are emitted using at least one acoustic transmitter fitted to an object and the difference(s) in the time(s) of arrival of the acoustic signals received by acoustic receivers are calculated. This difference in the time(s) of arrival of the acoustic signals is commonly called "Time Difference of Arrival" (TDoA) in English. The relative position of the acoustic receivers and the speed of propagation of the acoustic signals being known, it is then possible to determine position data of the acoustic transmitter and therefore of the object.

[0004] However, the precise determination of arrival times by known methods is made complex, or even impossible, in environments conducive to the reverberation of acoustic signals. For example, in an application to a charging robot used to charge an electric accumulator, the robot is configured to locate an inductor mounted on the underbody of the electric motor vehicle. The charging robot then moves under the vehicle. The acoustic signals are then emitted under the underbody of the vehicle where they are strongly subject to reverberations.

[0005] The reverberations of the acoustic signals behave as additional virtual sources for the acoustic receivers. Also, the signal received by an acoustic receiver in such a reverberant environment comprises several occurrences of the emitted signal, namely one occurrence corresponding to the directly received acoustic signal and one or more occurrences corresponding to the reverberated acoustic signal. These occurrences arrive successively at each acoustic receiver and together constitute the received signal. Therefore, taking into account these reverberations, it is It is particularly difficult to discriminate within the received acoustic signal the occurrence corresponding to a reverberated signal compared to the occurrence corresponding to the directly received emitted acoustic signal. Determining the times of arrival of the received acoustic signals then proves complex and consequently imprecise, which compromises the location of the object.

[0006] Methods according to the prior art are known using an identification technique implementing a cross-correlation between the emitted signal and the signals received by the acoustic receivers. A disadvantage of these methods is that they erroneously assume that within a received acoustic signal, the occurrence of higher amplitude corresponds to the emitted acoustic signal directly received. Indeed, the amplitude of the occurrences corresponding to the reverberated acoustic signals received by the acoustic receivers may be greater than that of the occurrence corresponding to the acoustic signal coming directly from the acoustic transmitter. This is due in particular to the directional sensitivity inherent in acoustic receivers which leads them to better perceive reverberated acoustic signals in a direction which is favorable to them, as well as to the directional sensitivity of the acoustic transmitters.

[0007] Consequently, these methods according to the prior art, implementing cross-correlation techniques, lead to determining arrival times from occurrences corresponding to reverberated acoustic signals, which induces inaccuracies in the calculation of the differences in arrival times and therefore significant errors in the location of the object. Statement of the invention

[0008] An aim of the present invention is to propose a method for locating an object which overcomes the aforementioned drawbacks.

[0009] To do this, the invention relates to a method for locating an object, the method comprising the steps according to which: - an object is provided equipped with at least one acoustic transmitter configured to emit acoustic signals; - at least one first acoustic receiver and one second acoustic receiver remote from the first acoustic receiver are provided, said first and second acoustic receivers being configured to receive acoustic signals; - at least one acoustic signal is emitted using said acoustic transmitter; - for each of the acoustic receivers, the following steps are carried out: upon receipt of an acoustic signal by an acoustic receiver, a cross-correlation is carried out between the acoustic signal received by said acoustic receiver and a reference signal, so as to determine a correlation signal; a transformation is applied to said determined correlation signal, so as to determine a transformed correlation signal whose shape follows the envelope of the correlation signal; we determine the set of local maxima of the transformed correlation signal prior to a predetermined final instant, within a predetermined time window; we determine an arrival time of the received acoustic signal, from the first local maximum of the transformed correlation signal considered within said predetermined time window, the method further comprising the steps of: - the difference(s) in arrival times between said acoustic signals received by said acoustic receivers are calculated from said arrival times of said previously determined received acoustic signals; - position data of the object are determined from said difference(s) in arrival times.

[0010] Said acoustic transmitter constitutes a source of acoustic signals. Preferably, the acoustic transmitters are ultrasonic transmitters. Preferably, said at least one acoustic signal emitted using said acoustic transmitter is an ultrasonic signal.

[0011] An acoustic signal is a sound signal. The emitted acoustic signal is preferably sinusoidal. The acoustic signal advantageously has peaks of different amplitudes. In a non-limiting manner, the emitted acoustic signal can have any shape. Without departing from the scope of the invention, the emitted acoustic signal can be a pulse.

[0012] Said at least one first and one second acoustic receivers preferably comprise microphones. The acoustic receivers are preferably fixed relative to each other. The relative position of said acoustic receivers is advantageously known at all times. Preferably, said acoustic receivers are mounted on the same support, for example on a robot.

[0013] Preferably, at least three acoustic receivers are provided, more preferably at least four acoustic receivers, more preferably five acoustic receivers. One advantage is to collect more position information of the object and thus improve the accuracy of the location of the object.

[0014] The steps of performing a cross-correlation, applying a transformation to the correlation signal, determining the set of local maxima of the transformed correlation signal and determining an arrival time of the received acoustic signal are carried out upon receipt of an acoustic signal by each of the acoustic receivers.

[0015] The signal received by an acoustic receiver comprises at least one occurrence cor corresponding to the acoustic signal emitted by the acoustic transmitter and directly received. In a non-limiting manner, in a reverberant environment, the acoustic signal received by an acoustic receiver comprises a plurality of occurrences, namely an occurrence corresponding to the directly received emitted signal and one or more occurrences corresponding to reverberations of the emitted acoustic signal. It is understood that the occurrences corresponding to the reverberations are received after the occurrence corresponding to the directly received emitted acoustic signal, without reverberation. All of said occurrences constitute the received acoustic signal.

[0016] The occurrences may have different amplitudes. In particular, the occurrence corresponding to the emitted acoustic signal may have a lower amplitude than the occurrences corresponding to reverberations of the emitted signal.

[0017] It is understood that the steps of performing a cross-correlation, applying a transformation to the correlation signal, determining local maxima and determining an arrival time are performed upon reception of acoustic signals by each of the acoustic receivers, so that a correlation signal, a transformed correlation signal, local maxima and arrival times are determined for each of the acoustic receivers, upon reception of an acoustic signal.

[0018] These steps aim in particular to identify, within the acoustic signal received by an acoustic receiver, which occurrence corresponds to the directly received emitted signal, without reverberation.

[0019] Cross-correlation is also called inter-correlation or "cross-correlation" in English. It makes it possible to measure the similarity between a reference signal and the acoustic signal received by an acoustic receiver and thus facilitate the identification of occurrences of the emitted acoustic signal within the received acoustic signal, particularly in the presence of noise. The amplitude of the correlation signal is maximum at times when the received acoustic signal is similar to the reference signal.

[0020] The correlation signal is preferably calculated according to the formula:

[0021] [Math.l] = f~ x( t+T ) y ( t ) dt

[0022] Where Sxy(r) is the correlation signal, r is a time variable, t is another time variable, x(t) is the reference signal and y(t) is the acoustic signal received by the corresponding acoustic receiver.

[0023] In a non-limiting manner, said transformation applied to the correlation signal may be filtering, a Hilbert transformation or any other transformation making it possible to obtain a transformed correlation signal whose shape follows the envelope of the correlation signal.

[0024] According to the invention, the application of a transformation to the correlation signal makes it possible to obtain a transformed correlation signal whose shape follows the general shape of the envelope of the corresponding correlation signal. One advantage is to obtain a transformed correlation signal whose shape is smoothed compared to that of the correlation signal. The transformed correlation signal has a reduced number of local maxima compared to the correlation signal. The determination of local maxima from said transformed correlation signal is therefore particularly simplified. Furthermore, each of the local maxima therefore corresponds to a local maximum for the associated occurrence. The identification of the occurrence corresponding to the directly received emitted acoustic signal is therefore facilitated.

[0025] Preferably, the shape of the transformed correlation signal follows the upper envelope of the correlation signal.

[0026] Preferably, said transformation is applied to said correlation signal taken as an absolute value.

[0027] In a non-limiting manner, the transformed correlation signal may correspond to the envelope of the determined correlation signal. In which case, the transformation applied is an envelope function determining the envelope of the correlation signal.

[0028] By local maximum is meant a maximum amplitude of the transformed correlation signal considered within a reduced time interval, which does not necessarily correspond to the global maximum of the transformed correlation signal considered over the whole of said transformed correlation signal.

[0029] Preferably, the local maxima of the transformed correlation signal correspond to the peaks of said transformed correlation signal.

[0030] The local maxima are preferably determined point to point. Preferably, an amplitude value of the transformed correlation signal is considered as a local maximum if it corresponds to a maximum amplitude of the transformed correlation signal considered within a reduced time interval, preferably of the order of the duration of the acoustic signal emitted by the acoustic transmitter.

[0031] Generally, but in a non-limiting manner, each occurrence of the emitted acoustic signal within the transformed correlation signal corresponds to a local maximum. By way of non-limiting example, in the case of a received acoustic signal comprising one occurrence corresponding to the directly received emitted acoustic signal and two occurrences corresponding to reverberations of the emitted acoustic signal, said transformed correlation signal advantageously comprises three local maximums.

[0032] According to the invention, at least all the local maxima of the transformed correlation signal prior to the predetermined final instant are determined. Without departing from the scope of the invention, the local maxima subsequent to said predetermined final instant could also be determined.

[0033] The predetermined final instant corresponds to an instant beyond which the occurrences within the acoustic signal received by the acoustic receiver cannot correspond to the signal directly received, without reverberation. It is therefore not necessary to determine the local maxima of the transformed correlation signal beyond said predetermined final instant.

[0034] The predetermined final time may be set in advance or depend on the acoustic signal received by the acoustic receiver. The predetermined final time advantageously depends on the acoustic signal received by the corresponding acoustic receiver. For each of the acoustic receivers, the predetermined final time is advantageously calculated after receiving an acoustic signal on the corresponding acoustic receiver. Preferably, the predetermined final time is different for the first and second acoustic receivers. Alternatively, and without departing from the scope of the invention, the predetermined final time may be identical for the first and second acoustic receivers.

[0035] Preferably, the predetermined time window extends between a predetermined initial time and said predetermined final time. The time window is advantageously chosen so as not to consider the local maxima of an acoustic signal received previously to the acoustic signal currently being processed.

[0036] By first local maximum of the transformed correlation signal considered within the predetermined time window is meant the local maximum of the transformed correlation signal occurring earliest within said predetermined time window. This first local maximum is considered to be associated with the occurrence, within the acoustic signal received by the acoustic receiver, corresponding to the emitted acoustic signal directly received, without reverberation. In other words, the determination of this first local maximum of the transformed correlation signal makes it possible to identify said first occurrence corresponding to the directly received signal in order to then be able to determine the time of arrival of the acoustic signal received from this occurrence.

[0037] The arrival time is considered as a reference time for the acoustic receiver, corresponding to the directly received emitted acoustic signal, and on the basis of which the calculation of arrival time differences is carried out. In a non-limiting manner, the arrival time of a received signal is not necessarily the start of the occurrence corresponding to the directly received emitted acoustic signal. The arrival time may for example be chosen as being the time corresponding to the maximum amplitude of said occurrence.

[0038] Preferably, for each of the acoustic receivers, the time of arrival of the received acoustic signal is calculated using the first local maximum of the transformed correlation signal considered within said predetermined time window. The arrival time of the acoustic signal is then not necessarily equal to the time of the first local maximum of the previously determined transformed correlation signal.

[0039] Preferably, but in a non-limiting manner, the time of arrival of the received acoustic signal is determined as being the time of a maximum of the untransformed correlation signal, close to said first maximum of the transformed correlation signal considered within said predetermined time window.

[0040] Preferably, for each of the acoustic receivers, a time range is determined centered on the first maximum of the transformed correlation signal, the time of arrival of the received acoustic signal being determined as being the time of the maximum of the untransformed correlation signal considered within this time range. The duration of this time range is preferably between one and two times the period of the emitted acoustic signal.

[0041] Alternatively, and without departing from the scope of the invention, for each of the acoustic receivers, the time of arrival of the received acoustic signal can be determined as being the time of the first local maximum of the previously determined transformed correlation signal considered within said predetermined time window.

[0042] Advantageously, when at least three acoustic receivers are provided, the differences in arrival times between the acoustic signals received by all of the acoustic receivers are calculated.

[0043] The position data of the object are preferably calculated from the determined arrival time difference(s), the known position of the acoustic receivers, the propagation speed of the acoustic signals and possibly other physical parameters.

[0044] The position data of the object are preferably position data in space, therefore in three dimensions, or in a plane, for example in a plane comprising the acoustic receivers.

[0045] Preferably, the use of two or three acoustic receivers makes it possible to calculate position data of the object in the plane while the use of four or five acoustic receivers makes it possible to calculate position data of the object in space. The position data of the object are preferably geographical coordinates determined in the terrestrial reference frame or relative to a reference element, for example relative to the acoustic receivers.

[0046] The method according to the invention makes it possible to effectively discriminate and identify the occurrences corresponding to the directly received emitted acoustic signal compared to the occurrences corresponding to the reverberations of the emitted acoustic signal. The method according to the invention therefore makes it possible to identify, within the received acoustic signal, which occurrence corresponds to the directly received emitted acoustic signal, without reverberation. beration and, therefore, to determine the time of arrival of the acoustic signal received from this occurrence.

[0047] Consequently, the invention makes it possible to standardize the determination of the differences in arrival times from the occurrences corresponding to the directly received emitted acoustic signal, without reverberation. The accuracy of the calculated position data is improved compared to the methods according to the prior art which assume that within a received acoustic signal, the occurrence of highest amplitude corresponds to the directly received emitted acoustic signal.

[0048] Preferably, for each pair of acoustic receivers, a maximum threshold for the difference in arrival times is determined and the object position data is determined from the calculated difference(s) in arrival times having a value less than or equal to said maximum threshold for the difference in arrival times. In other words, the calculated differences in arrival times having a value greater than said maximum threshold for the difference in arrival times are filtered and are not considered in the calculation of the object position data.

[0049] Preferably, for a given pair of acoustic receivers, the maximum threshold of difference in arrival times is calculated as being the ratio between the distance between the two acoustic receivers and the speed of propagation of the sound.

[0050] Advantageously, the reference signal corresponds to said acoustic signal emitted using said acoustic transmitter. The reference signal is therefore the emitted acoustic signal or an image of the emitted acoustic signal. One interest is to measure the similarity between the emitted acoustic signal and the received acoustic signal. The amplitude of the correlation signal is then maximum at the times corresponding to the occurrences associated with the emitted acoustic signal.

[0051] Preferably, the emitted acoustic signal is a wave train. One advantage is to be able to easily carry out a cross-correlation between the received acoustic signal and the reference signal, in particular when the reference signal corresponds to the emitted acoustic signal. This also facilitates the application of the transformation, for example of the low-pass filter, to the correlation signal and consequently the determination of the arrival times of the received signals, associated with the occurrence corresponding to the directly received emitted acoustic signals.

[0052] Advantageously, for each of the acoustic receivers, the arrival time of the received acoustic signal is determined from the first local maximum of the transformed correlation signal, considered within said predetermined time window, having an amplitude greater than a predetermined amplitude threshold. In other words, any local maximum having an amplitude less than or equal to the predetermined amplitude threshold, within said predetermined time window, is not taken into account for determining the arrival time. One advantage is not to consider irrelevant or insignificant local maxima of the transformed correlation signal, which would not be possible to distinguish from disturbing noise. Another interest is to improve the accuracy of the calculated object position data.

[0053] In a non-limiting manner, said predetermined amplitude threshold may be identical or different for each of the acoustic receivers. The amplitude threshold may be a function of measured physical parameters.

[0054] Advantageously, for each of the acoustic receivers, said predetermined amplitude threshold is a function of the amplitude value of the overall maximum of the corresponding transformed correlation signal.

[0055] Preferably, but not limited to, for a given acoustic receiver, the predetermined amplitude threshold is the maximum between a signal-to-noise ratio level measured for said acoustic receiver and a percentage of the amplitude value of the overall maximum of the transformed correlation signal.

[0056] The predetermined amplitude threshold may be chosen as a percentage of said amplitude value of the overall maximum of the transformed correlation signal.

[0057] Preferably, for each of the acoustic receivers, after having determined the correlation signal, the instant corresponding to the maximum of said correlation signal is determined, before transformation, this instant being chosen as the predetermined final instant. Also, only the local maxima prior to this instant of the maximum of the correlation signal are considered for determining the instant of arrival of the acoustic signal. One advantage is to reduce the search window for local maxima and therefore the resources required for the associated calculations. Indeed, the instant of the maximum of the correlation signal corresponds either to the occurrence corresponding to the directly received emitted acoustic signal, or to the occurrence corresponding to a reverberation of the emitted acoustic signal.Therefore, the following occurrences can only be occurrences corresponding to other reverberations of the emitted acoustic signal which should not be taken into account when determining the arrival times.

[0058] Preferably, said transformation is a low-pass filter applied to said correlation signal, taken as an absolute value. This filter makes it possible to obtain a transformed correlation signal whose shape follows the envelope of the correlation signal, and whose amplitude is less than that of said envelope.

[0059] It is understood that for the application of the low-pass filter, at each instant, the amplitude of the correlation signal is considered in absolute value. One advantage is not to obtain a substantially zero filtered correlation signal.

[0060] The low-pass filter is configured to pass frequencies below a cutoff frequency while attenuating frequencies above said cutoff frequency. Preferably, but not limited to, the applied low-pass filter is identical for each acoustic receiver.

[0061] Advantageously, the low-pass filter applied to the absolute value of the correlation signal determined for the acoustic signal received by each of the acoustic receivers has a cut-off frequency which is determined from the frequency of said acoustic signal emitted using said acoustic transmitter. Preferably, the cut-off frequency of the low-pass filter is less than or equal to the frequency of the emitted acoustic signal.

[0062] Preferably, a plurality of successive acoustic signals are emitted using said acoustic transmitter, and new position data of said object are determined from new acoustic signals received by the first and second acoustic receivers. One advantage is to calculate the position data of the object several times in order to have a panel of position data making it possible to determine the position of said object even more precisely. It is in particular possible to average these position data. Another advantage is to update and correct the position data of the object over time, in particular in the case where the acoustic receivers or the acoustic transmitter are moved in the plane or in space. This makes it possible to further improve the accuracy of said position data.

[0063] Said acoustic signals may be emitted at regular intervals or not. In a non-limiting manner, the acoustic signals may be emitted in bursts.

[0064] Preferably, an acoustic signal is emitted after the acoustic signal associated with the previous emitted acoustic signal has been received. A new acoustic signal is preferably emitted a few moments after the end of a received acoustic signal.

[0065] Preferably, for each of the acoustic receivers, the predetermined time window for a given received acoustic signal is determined so that it extends over a time period less than the time interval between the emission of an acoustic signal corresponding to said received acoustic signal and the emission of the preceding acoustic signal.

[0066] One advantage is not to consider the previously received acoustic signal when determining an arrival time of a given received acoustic signal, which would lead to inaccuracies, or even errors in determining said arrival time.

[0067] Preferably, for each of the acoustic receivers, upon reception of an acoustic signal by an acoustic receiver, the duration of the previously received acoustic signal is measured, and the predetermined time window for the received acoustic signal is determined so that it extends over a time period Tw substantially equal to:

[0068] [Math.2]

[0069] where Ti is the time interval between the emission of the previous acoustic signal and the emission of the acoustic signal corresponding to the received acoustic signal, and To is the measured duration of the previously received acoustic signal. Another advantage is to reduce the predetermined time window so as not to consider the previously received acoustic signal in determining the object's position data.

[0070] It is understood that the previously received acoustic signal is associated with a previously emitted acoustic signal, while the received acoustic signal, for which the predetermined time window is determined, is associated with the emitted acoustic signal.

[0071] Advantageously, at least one third acoustic receiver is further provided, remote from the first and second acoustic receivers and configured to receive acoustic signals, and for the third acoustic receiver the following steps are carried out: upon receipt of an acoustic signal by the third acoustic receiver, a cross-correlation is carried out between the acoustic signal received by said third acoustic receiver and a reference signal, so as to determine a correlation signal; a transformation is applied to said determined correlation signal, so as to determine a transformed correlation signal whose shape follows the envelope of the correlation signal; we determine the set of local maxima of the transformed correlation signal prior to a predetermined final instant, within a predetermined time window; we determine an arrival time of the acoustic signal received by the third acoustic receiver, from the first local maximum of the transformed correlation signal considered within said predetermined time window, the method further comprising the steps of: - the differences in arrival times between said acoustic signals received by said first, second and third acoustic receivers are calculated from said arrival times of said previously determined received acoustic signals; - position data of the object are determined from said differences in arrival times.

[0072] The steps of determining the arrival time are repeated for each of the three acoustic receivers. Also, upon transmission of an acoustic signal, three arrival times are determined, each being associated with an acoustic signal received on one of the three acoustic receivers.

[0073] It is understood that the difference in arrival times is calculated between the arrival time of the acoustic signal received on the first acoustic receiver and the arrival time of the acoustic signal received on the second acoustic receiver, as well as the difference in arrival times between the arrival time of the acoustic signal received on the second acoustic receiver and the arrival time of the acoustic signal received on the third acoustic receiver, but also the difference in arrival times between the arrival time of the acoustic signal received on the first acoustic receiver and the arrival time of the acoustic signal received on the third acoustic receiver.

[0074] By increasing the number of acoustic receivers and therefore the number of calculated arrival time differences, the accuracy of the calculated object position data is improved.

[0075] Preferably, calculating a difference in arrival times between the acoustic signals received by two acoustic receivers makes it possible to define a hyperbola in the plane or in space on which the object can be positioned. Using three acoustic receivers to determine three differences in arrival times then makes it possible to define three hyperbolas.

[0076] The number of defined hyperbolas is equal to N*(Nl) / 2, where N is the number of acoustic receivers.

[0077] These hyperbolas advantageously have at least one intersection corresponding to the position of the object.

[0078] Hyperbolas may have multiple intersections. The object's position data are then determined by solving an optimization problem. Each hyperbola can be assigned a weighting to facilitate the selection of the most realistic intersection point.

[0079] In the absence of intersection, the position of the object can be chosen as being the closest point of the set of hyperbolas, each hyperbola being able to be weighted again.

[0080] Preferably, a mobile robot is provided comprising a movement system, said first and second acoustic receivers being mounted on said mobile robot, and the mobile robot is moved towards said object using the determined position data of said object.

[0081] Said mobile robot may be a charging robot configured to charge an electric accumulator of an electric motor vehicle. The charging robot preferably comprises a primary inductor and said acoustic transmitter is preferably disposed on a secondary inductor mounted under the vehicle and electrically connected to said electric accumulator.

[0082] The invention further relates to a device for locating an object, the locating device comprising: - an acoustic transmitter configured to be mounted on said object to be located, the acoustic transmitter being configured to emit acoustic signals; - at least a first acoustic receiver and a second acoustic receiver, said first and second acoustic receivers being distant from each other and configured to receive acoustic signals; - a control module configured to control said acoustic transmitter so as to emit at least a first acoustic signal; - a processing unit comprising: a correlation module configured to carry out, for each of the acoustic receivers, upon receipt of an acoustic signal by an acoustic receiver, a cross-correlation between the acoustic signal received by said acoustic receiver and a reference signal, so as to determine a correlation signal; a transformation module configured to apply, for each of the acoustic receivers, a transformation to said determined correlation signal, so as to determine a transformed correlation signal whose shape follows the envelope of the correlation signal; a first determination module configured to determine, for each of the acoustic receivers, all of the local maximums of the transformed correlation signal prior to a predetermined end, within a predetermined time window; a second determination module configured to determine, for each of the acoustic receivers, an arrival time of the received acoustic signal, from the first local maximum of the transformed correlation signal considered within said predetermined time window, a first calculator configured to calculate the difference(s) in arrival times between the acoustic signals received by acoustic receivers from said arrival times of said acoustic signals; a second computer configured to determine position data of the object from said difference(s) in arrival times.

[0083] The invention also relates to an installation comprising: - an object to locate; - a mobile robot comprising a movement system; - a location device as described above, said at least one first acoustic receiver and one second acoustic receiver being mounted on said mobile robot, said acoustic transmitter being mounted on said object, the movement system being configured to move the mobile robot towards said object from position data of said object provided by the location device.

[0084] Preferably, the installation further comprises an electric motor vehicle comprising an electric accumulator, the mobile robot being a charging robot provided with of a primary inductor and said object being a secondary inductor mounted on said vehicle and electrically connected to the electric accumulator, the primary inductor being configured to be placed opposite the secondary inductor to allow the transmission of electrical power from the primary inductor to the secondary inductor, by magnetic coupling, in order to charge said electric accumulator. The location device makes it possible to calculate position data of the secondary inductor, thus making it possible to guide the charging robot to said secondary inductor, so as to place the primary inductor opposite the secondary inductor.

[0085] The secondary inductor is preferably mounted on an underbody of the electric vehicle, so that the mobile robot is configured to position itself under said secondary inductor, under the vehicle, so as to charge the electric accumulator. Brief description of the drawings

[0086] The invention will be better understood on reading the following description of an embodiment of the invention given by way of non-limiting example, with reference to the appended drawings, in which:

[0087] [Fig.l] [Fig.l] is a diagram illustrating a location device according to the invention;

[0088] [Fig.2] [Fig.2] is an installation comprising a location device according to the invention;

[0089] [Fig.3] [Fig.3] shows an acoustic signal emitted using an acoustic transmitter of the locating device of [Fig.l];

[0090] [Fig.4] [Fig.4] shows the acoustic signals received by the first and second acoustic receivers of the locating device of [Fig.l];

[0091] [Fig.5] [Fig.5] shows correlation signals determined from said acoustic signals received by the first and second acoustic receivers;

[0092] [Fig.6] [Fig.6] shows transformed correlation signals obtained by transforming said correlation signals;

[0093] [Fig.7] [Fig.7] shows the determination of an arrival time from the first maximum of the transformed correlation signal;

[0094] [Fig.8] [Fig.8] illustrates the difference in arrival times calculated by means of the first calculation module;

[0095] [Fig.9] [Fig.9] illustrates the determination of position data of the object using the second calculation module; and

[0096] [Fig.l0] [Fig. 10] shows the steps of an embodiment of the location method according to the invention. Description of the embodiments

[0097] The invention relates to a method for acoustically locating an object implemented by means of a tracking device.

[0098] [Fig. 1] illustrates a general diagram of a device 10 for locating an object O according to the invention. The locating device 10 makes it possible to determine position data of the object O. The object O may be an inductor connected to an electric accumulator of an electric vehicle. In this non-limiting example, the locating device 10 comprises an acoustic transmitter 12 configured to emit acoustic signals. The acoustic transmitter 12 is mounted on the object O.

[0099] The location device 10 further comprises a first acoustic receiver 14, a second acoustic receiver 16 and a third acoustic receiver 18. The first, second and third acoustic receivers 14, 16, 18 are mounted on the same support S. They are separated from each other by a fixed and known distance. The acoustic receivers are configured to receive acoustic signals.

[0100] The locating device 10 also comprises a control module 100 configured to control the acoustic transmitter 12 in order to emit acoustic signals.

[0101] The location device 10 further comprises a processing unit 20 with which the acoustic receivers 14, 16, 18 communicate. The processing unit 20 comprises a correlation module 22 configured to carry out a cross-correlation between each of the acoustic signals that it receives as input and a reference signal x(t), and thus determine correlation signals.

[0102] The processing unit 20 comprises a transformation module 24, configured to apply a transformation to the correlation signals provided by the correlation module 22, and thus deliver transformed correlation signals.

[0103] The processing unit 20 further comprises a first determination module 26 configured to determine the local maxima of the transformed correlation signals provided by the transformation module 24. The processing unit 20 further comprises a second determination module 28 configured to determine times of arrival of the received acoustic signals from the local maxima determined by the first determination module 26.

[0104] The processing unit 20 further comprises a first calculator 30 configured to calculate the differences in arrival times between the received acoustic signals. The processing unit 20 finally comprises a second calculator 32 configured to determine position data of the object O from said differences in arrival times.

[0105] An embodiment of the method for locating an object according to the invention will now be described with reference to Figures 2 to 10. The method is implemented by means of the locating device of [Fig.l].

[0106] [Fig.2] illustrates an installation 8 according to the invention comprising a vehicle with motor V equipped with an electric accumulator (not shown). The installation 8 further comprises a mobile robot, in this case a charging robot R. The charging robot R comprises a movement system W comprising wheels. The charging robot further comprises a primary inductor Ii. The vehicle V further comprises a secondary inductor I2 electrically connected to the electric accumulator of the motor vehicle and mounted on the underbody Vo of the vehicle. The primary inductor Ix is configured to be brought opposite the secondary inductor I2 to achieve magnetic coupling and allow the transmission of electrical power from the primary inductor Ix to the secondary inductor I2 so as to charge the electric accumulator. The secondary inductor I2 here forms an object to be located.

[0107] According to the invention, in a first step S1, an object to be located is provided, in this case the secondary inductor I2. The object to be located is provided with said acoustic transmitter 12. In a second step S2, the first, second and third acoustic receivers 14, 16, 18 are also provided. Said acoustic receivers are mounted on the loading robot R.

[0108] In a third step S3, a plurality of successive acoustic signals are emitted in bursts. In this non-limiting example, for clarity, we will only consider the emission of a single acoustic signal x(t) using the acoustic transmitter 12. The emitted signal x(t) is illustrated in [Fig.3]. The emission of this acoustic signal x(t) is controlled using the control module 100. The emitted acoustic signal x(t) is here a wave train. It is sinusoidal. This acoustic signal x(t) is an ultrasonic signal.

[0109] As can be seen in [Fig. 2], the emitted acoustic signal x(t) is received directly on each of the first, second and third acoustic receivers 14, 16, 18. However, the emitted acoustic signal also undergoes reverberations on the underbody Vq of the vehicle V. The reverberated acoustic signals are also received by each of the acoustic receivers. The first acoustic receiver 14 receives an acoustic signal y,(t) resulting from the sum of the directly received emitted acoustic signal and the reverberated acoustic signals. The second acoustic receiver 16 receives an acoustic signal y2(t) resulting from the sum of the directly received emitted acoustic signal and the reverberated acoustic signals. The third acoustic receiver 18 receives an acoustic signal y3(t) resulting from the sum of the directly received emitted acoustic signal and the reverberated acoustic signals.These acoustic signals are received at given times by the acoustic receivers, these times being able to be identical or different. Each of these acoustic signals yi(t),y2(t),y3(t) received is associated with an arrival time tOi,tO2,to3 which must be determined in order to then be able to determine position data of the object O. These arrival times tOi,tO2,to3 can be different from each other.

[0110] [Fig.4] illustrates in solid lines the acoustic signal y,(t) received by the first acoustic receiver 14 and, in dotted lines, the acoustic signal y2(t) received by the second acoustic receiver 16. The signal received by the third acoustic signal y3(t) is not illustrated here for clarity. [Fig.4] shows the amplitude A of the received acoustic signals yi(t),y2(t) as a function of time t. It can be seen that the signal y,(t) received by the first acoustic receiver 14 has a first occurrence Oi corresponding to the directly received emitted acoustic signal, a second occurrence O2 corresponding to a first reverberation of the emitted acoustic signal and a third occurrence O3 corresponding to a second reverberation of the emitted acoustic signal x(t). The arrival time tOi of the acoustic signal y,(t) received by the first acoustic receiver 14 must be determined from the occurrence corresponding to the directly received emitted acoustic signal, without reverberation.However, it is not trivial to determine what this occurrence is.

[0111] In particular, as can be seen in [Fig.4], the amplitude A of the second occurrence O2 is greater than that of the first occurrence Oi even though this second occurrence corresponds to a reverberation of the emitted signal. This is due in particular to the directional sensitivity of the acoustic receivers. It is therefore not possible to presuppose that the occurrence of higher amplitude corresponds to the emitted acoustic signal directly received. The method according to the invention makes it possible to identify which occurrence corresponds to the acoustic signal directly received, without reverberation.

[0112] To do this, according to the invention, in a fourth step S4, upon receipt of acoustic signals, and for each of the acoustic receivers 14, 16, 18, a cross-correlation is then carried out between the received acoustic signal yi(t), y2(t), y3(t) and a reference signal, here the emitted acoustic signal x(t). A correlation signal Sxyi(t), Sxy2(t), Sxy3(t) is thus determined for each acoustic receiver. This step is implemented by means of the correlation module 22. The correlation signals Sxyi(t), Sxy2 (t) determined for the first and second acoustic receivers 14, 16 are illustrated in [Fig.5].

[0113] It is noted that the amplitude maximums of the correlation signal Sxyi(t) determined for the first acoustic receiver 14 correspond to the second occurrence O2 corresponding to a reverberation of the acoustic signal x(t) emitted.

[0114] According to the invention, in a fifth step S5, for each of the acoustic receivers 14, 16, 18 a transformation is applied to the previously determined correlation signals in order to determine three transformed correlation signals each having a shape which follows the envelope of the corresponding correlation signal. In this non-limiting example, the transformation consists of applying a low-pass filter to the previously determined correlation signals, considered in value absolute. The low-pass filter applied is here identical for the three acoustic receivers. The low-pass filter has a cut-off frequency substantially equal to the frequency of the acoustic signal x(t) emitted using the acoustic transmitter 12. Three transformed correlation signals are thus determined. The low-pass filter is applied by means of the transformation module 24. The result of this transformation, and more precisely of this filtering, is illustrated in [Fig.6], where only the transformed correlation signals Sxyif(t),Sxy2f(t),Sxy3f(t) determined for the first and second acoustic receivers are represented.

[0115] As can be seen in [Fig.6], the transformed correlation signals Sxyif(t),Sxy2f( t) have a shape that follows the general shape of the envelope of the corresponding correlation signal Sxyi(t),Sxy2(t). The determination of local maxima from the transformed correlation signals is facilitated.

[0116] According to the invention, in a sixth step S6, the set of local maxima Mi,M2 of the transformed correlation signals Sxyif(t),Sxy2f(t) which are prior to a predetermined final instant tt, within a predetermined time window, is then determined.

[0117] For each acoustic receiver, the predetermined final instant tt is here chosen as being the instant corresponding to the global maximum of the corresponding correlation signal Sxyi(t),Sxy2(t), Sxy3(t). In this non-limiting example, only two local maximums are determined insofar as the second occurrence within the received acoustic signal is that of the highest amplitude.

[0118] The predetermined time window has a predetermined time period Tw extending between a predetermined initial instant tj and said predetermined final instant t f-

[0119] Each transformed correlation signal of each of the acoustic receivers is associated with a predetermined final instant, a predetermined initial instant and a predetermined time window. For reasons of clarity, only the predetermined final instant tf, the predetermined initial instant tj and the predetermined time window associated with the transformed correlation signal Sxyif(t) of the first acoustic receiver are illustrated in [Fig.6].

[0120] Only the local maxima Mi,M2 of the transformed correlation signals Sxyi(t),Sxy2f(t),Sxy3f(t) are determined for each of the acoustic receivers, within the predetermined time window. The determination of said local maxima Mi,M2 is done point by point. These local maxima are illustrated in [Fig.6].

[0121] According to the invention, in a seventh step S7, an arrival time tOi,tO2,to3 is then determined for each of the acoustic signals received yi(t),y2(t),y3(t) by each of the first, second and third acoustic receivers 14,16,18. These arrival times tOi,tO2,to3 are determined from the first local maximum Mi of the transformed correlation signal Sxyi(t),Sxy2f(t),Sxy3f(t) considered within said predetermined time window which has an amplitude greater than a predetermined amplitude threshold Ao. This first local maximum Mi is the maximum occurring earliest within the predetermined time window and which has an amplitude greater than the predetermined amplitude threshold Ao.

[0122] For an acoustic signal received by a given acoustic receiver, the predetermined amplitude threshold Ao is determined as being the maximum between a signal-to-noise ratio level measured for said acoustic receiver and a percentage of the amplitude value of the overall maximum of the transformed correlation signal Sxyif(t), corresponding here to M2.

[0123] [Fig.7] illustrates the determination of an arrival time tOi from said first local maximum Mi of the transformed correlation signal. In this non-limiting example, a first corrected maximum Mic is determined from said first maximum Mi of the transformed correlation signal. To do this, as illustrated in [Fig.7], a time range Tz is determined centered on the first maximum Mi of the transformed correlation signal Sxyi(t),Sxy2f(t),Sxy3f(t). The arrival time toi of the received acoustic signal is then determined as being the time of the maximum of the untransformed correlation signal considered within this time range Tz. This maximum of the correlation signal considered within the time range forms a first corrected maximum Mic. This corrected maximum Mic is the maximum of the correlation signal closest to said first maximum Mi of the transformed correlation signal.

[0124] The first local maximum Mi is considered to be associated with the occurrence, within the acoustic signal received by the acoustic receiver, corresponding to the directly received emitted acoustic signal, without reverberation. The invention therefore makes it possible to determine arrival times toi,to2,to3 for the received acoustic signals yi(t),y2(t),y3(t) which are associated with the first occurrence corresponding to the directly received emitted acoustic signal, without reverberation.

[0125] In an eighth step S8, the differences in arrival times TDOA12, TDOA2.3, TDOAi.3 between said acoustic signals received by said acoustic receivers are then calculated from said arrival times toi, to2, to3 of said previously determined received acoustic signals.

[0126] In this non-limiting example, the difference in arrival times TDOAi.2 between the acoustic signal yi(t) received by the first acoustic receiver 14 and the acoustic signal y2(t) received by the second acoustic receiver 16 is calculated according to: TDOAj.2 = to2-toi. In this non-limiting example, the difference in arrival times TDOA2.3 between the acoustic signal y2(t) received by the second acoustic receiver 16 and the acoustic signal y3(t) received by the third acoustic receiver 18 is calculated according to: TDOA2.3 = to3-to2. In this non-limiting example, the difference in arrival time TDOAi.3 between the acoustic signal y ,(t) received by the first acoustic receiver 14 and the acoustic signal y3(t) received by the third acoustic receiver 18 is calculated according to: TDOAi.3 = to3-toi.

[0127] In a non-limiting manner, these differences in arrival times can be filtered or weighted.

[0128] [Fig. 8] shows a comparison between the calculation of the difference in arrival times TDOAi.2 between the acoustic signals received by the first and second acoustic receivers 14, 16 implemented by the method according to the invention and this same difference in arrival times TDOAi.2' calculated by means of a method according to the prior art. It can be seen that the method according to the prior art incorrectly assumes that within the received acoustic signal yi(t),y2(t),y3(t), the occurrence of highest amplitude corresponds to the directly received emitted signal. Therefore, the arrival times t oi'doz' are incorrectly determined based on the occurrence of highest amplitude, here the second occurrence O2, which corresponds to a reverberation of the emitted acoustic signal. The position data which are then determined are then particularly imprecise.

[0129] On the contrary, in the method according to the invention, the arrival times tOi,tO2,to3 of the received acoustic signals are determined with respect to the occurrence Oi corresponding to the directly received emitted acoustic signal, without reverberation. The accuracy of the arrival time differences and therefore of the calculated position data is improved.

[0130] As illustrated in [Fig.9], in a ninth step S9, from said differences in arrival times TDOAi.2, TDOA2.3, TDOAi.3 previously determined for each of the acoustic receivers, position data Gx, Gy, Gz of the object in the plane, for example in the plane of the acoustic receivers, or in space, are determined. Said position data are also calculated as a function of the distance between the acoustic receivers 14, 16, 18 and the propagation speed of the acoustic signals in the measurement environment.

[0131] To determine said object position data, in this non-limiting example, hyperbolas are determined, in particular from said differences in arrival times TDOAi.2, TDOA2.3, TDOAi.3 previously determined. The intersection(s) of the hyperbolas are then determined. Each of the hyperbolas can then be weighted to enable the selection of the most relevant intersection. The coordinates Gx, Gy, Gz of the object in the plane are then determined from these hyperbolas.

[0132] In a non-limiting manner, any differences in arrival times whose value is greater than a maximum threshold for difference in arrival times are filtered and are not considered in determining the position data of the object. Said maximum threshold for arrival time is advantageously calculated as being the ratio between the distance between the two acoustic receivers and the speed of sound propagation.

[0133] [Fig. 10] shows the steps of the mode of implementation of the location method according to the invention described previously.

Claims

1.

2.

3. Claims Method for locating an object (O), the method comprising the steps of: - an object is provided equipped with at least one acoustic transmitter (12) configured to emit acoustic signals; - at least one first acoustic receiver (14) and one second acoustic receiver (16) remote from the first acoustic receiver are provided, said first and second acoustic receivers being configured to receive acoustic signals; - at least one acoustic signal (x(t)) is emitted using said acoustic transmitter; - for each of the acoustic receivers, the following steps are carried out: upon receipt of an acoustic signal (yi(t),y2(t),y3(t)) by an acoustic receiver, a cross-correlation is carried out between the acoustic signal received by said acoustic receiver and a reference signal, so as to determine a correlation signal (Sxyi(t),Sxy2(t),Sxy3(t)); a transformation is applied to said determined correlation signal, so as to determine a transformed correlation signal (Sxyif(t),Sxy2f (t),Sxy3f(t)) whose shape follows the envelope of the correlation signal; determining the set of local maxima (MbM2) of the transformed correlation signal (Sxyif(t),Sxy2f(t),Sxy3f(t)) prior to a predetermined final instant (tf), within a predetermined time window; determining an arrival instant (toi,to2,to3) of the received acoustic signal, from the first local maximum (Mi) of the transformed correlation signal considered within said predetermined time window, the method further comprising the steps according to which: - the difference(s) in arrival times (TDOAi 2,TDOA 2_3,TDOAi_3) between said acoustic signals received by said acoustic receivers are calculated from said arrival times of said previously determined received acoustic signals; - position data (Gx, Gy, Gz) of the object are determined from said difference(s) in arrival times. A localization method according to claim 1, wherein said reference signal corresponds to said acoustic signal emitted (x(t)) using said acoustic transmitter (12). A method of locating according to claim 1 or 2, wherein said emitted acoustic signal (x(t)) is a wave train.

4. Localization method according to any one of claims 1 to 3, in which, for each of the acoustic receivers (14, 16, 18), the arrival time (t0i, t02, t03) of the received acoustic signal (yi(t), y 2(t), y3(t)) is determined from the first local maximum (MJ of the transformed correlation signal (Sxyif(t), Sxy2f(t), Sxy3f(t)), considered within said predetermined time window, having an amplitude greater than a predetermined amplitude threshold (Ao).

5. A localization method according to claim 4, wherein, for each of the acoustic receivers (14,16,18), said predetermined amplitude threshold (Ao) is a function of the amplitude value of the global maximum (M2) of the corresponding transformed correlation signal (Sxyif(t),Sxy2f(t),Sxy3f(t)).

6. Location method according to any one of claims 1 to 5, in which, for each of the acoustic receivers (14, 16, 18), after having determined the correlation signal (Sxyi(t), Sxy2(t), Sxy3(t)), the instant corresponding to the maximum of said correlation signal is determined, before transformation, this instant being chosen as the predetermined final instant (tf).

7. A localization method according to any one of claims 1 to 6, wherein said transformation is a low-pass filter applied to said correlation signal (Sxyi(t),Sxy2(t),Sxy3(t)), taken as an absolute value.

8. A localization method according to claim 7, wherein the low-pass filter applied to the absolute value of the correlation signal (Sxyi(t),S xy2(t),Sxy3(t)) determined for the acoustic signal received (yi(t),y2(t),y3(t)) by each of the acoustic receivers has a cut-off frequency which is determined from the frequency of said acoustic signal emitted (x(t)) using said acoustic transmitter.

9. Location method according to any one of claims 1 to 8, in which a plurality of successive acoustic signals are emitted using said acoustic transmitter, and in which new position data (Gx, Gy, Gz) of said object (0) are determined from new acoustic signals received by the first and second acoustic receivers.

10. A localization method according to claim 9, wherein, for each of the acoustic receivers, the predetermined time window for a given received acoustic signal (yi(t),y2(t),y3(t)) is determined such that it extends over a time period less than the interval time between the emission of an acoustic signal corresponding to said received acoustic signal and the emission of the previous acoustic signal.

11. A localization method according to any one of claims 1 to 10, wherein at least one third acoustic receiver is further provided, remote from the first and second acoustic receivers and configured to receive acoustic signals, and for the third acoustic receiver, the following steps are carried out: upon receipt of an acoustic signal (y3(t)) by the third acoustic receiver, a cross-correlation is carried out between the acoustic signal received by said third acoustic receiver and a reference signal, so as to determine a correlation signal (Sxy3(t)); a transformation is applied to said determined correlation signal, so as to determine a transformed correlation signal (Sxy3f(t)) whose shape follows the envelope of the correlation signal;determining the set of local maxima of the transformed correlation signal (Sxy3f(t)) prior to a predetermined final instant, within a predetermined time window; determining an arrival time (t03) of the acoustic signal received by the third acoustic receiver, from the first local maximum of the transformed correlation signal considered within said predetermined time window, the method further comprising the steps according to which: - calculating the differences in arrival times (TDOAi 2,TDOA2 3,TDOAi 3) between said acoustic signals received by said first, second and third acoustic receivers from said arrival times of said previously determined received acoustic signals; - determining position data (Gx,Gy,Gz) of the object (0) from said differences in arrival times.;

12. A localization method according to any one of claims 1 to 11, wherein a mobile robot is provided comprising a movement system (W), said first and second acoustic receivers being mounted on said mobile robot, and wherein the mobile robot is moved towards said object using the determined position data (Gx, Gy, Gz) of said object.

13. Device (10) for locating an object (0), the locating device comprising: - an acoustic transmitter (12) configured to be mounted on said object (0) to be located, the acoustic transmitter being configured to emit acoustic signals; - at least one first acoustic receiver (14) and one second acoustic receiver (16), said first and second acoustic receivers being distant from each other and configured to receive acoustic signals; - a control module (100) configured to control said acoustic transmitter so as to emit at least a first acoustic signal (yi(t),y2(t),y3(t)); - a processing unit (20) comprising: a correlation module (22) configured to carry out, for each of the acoustic receivers, upon receipt of an acoustic signal by an acoustic receiver, a cross-correlation between the acoustic signal received by said acoustic receiver and a reference signal, so as to determine a correlation signal (Sxyi(t),Sxy2(t),Sxy3(t)); a transformation module (24) configured to apply, for each of the acoustic receivers, a transformation to said determined correlation signal, so as to determine a transformed correlation signal (Sxyif(t),Sxy2f(t),Sxy3f(t)) whose shape follows the envelope of the correlation signal; a first determination module (26) configured to determine, for each of the acoustic receivers, the set of local maxima (MbM2) of the transformed correlation signal (Sxyif(t),Sxy2f(t),Sxy3f (t)) prior to a predetermined final instant (tf), within a predetermined time window; a second determination module (28) configured to determine, for each of the acoustic receivers, an arrival time (to,to2,to3) of the received acoustic signal, from the first local maximum (MJ) of the transformed correlation signal considered within said predetermined time window, a first calculator (30) configured to calculate the difference(s) of arrival times (TDOAi 2,TDOA2 3,TDOAi 3) between the acoustic signals received by the acoustic receivers from said arrival times of said acoustic signals; a second calculator (32) configured to determine position data (Gx, Gy, Gz) of the object from said difference(s) in arrival times.

14. Installation comprising: - an object to be located (O); - a mobile robot (R) comprising a movement system; - a localization device (10) according to claim 13, said at least one first acoustic receiver (14) and one second acoustic receiver (16) being mounted on said mobile robot, said acoustic transmitter (12) being mounted on said object, the movement system (W) being configured to move the mobile robot towards said object from position data (Gx, Gy, Gz) of said object provided by the localization device.

15. Installation according to claim 14, further comprising a vehicle (V) with an electric motor comprising an electric accumulator, the mobile robot (R) being a charging robot provided with a primary inductor (IJ and said object (O) being a secondary inductor (I2) mounted on said vehicle and electrically connected to the electric accumulator, the primary inductor being configured to be placed opposite the secondary inductor to allow the transmission of electrical power from the primary inductor to the secondary inductor, by magnetic coupling, in order to charge said electric accumulator.

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