EP2143080B1 - Method for detecting an identification object in a vehicle - Google Patents

Description

Field of the invention

The present invention relates to a method for detecting an identification object in an area around an antenna device (s) and a detection system implementing the detection method.

It finds a particular application for a motor vehicle equipped with a hands-free detection system.

State of the art

According to a known state of the art, there is a method of detecting an object of identification such as a badge which acts as a receiver-transmitter to know if it is inside or outside of the passenger compartment of the vehicle. If the badge is inside the vehicle, the user is allowed to start the vehicle.

The detection of the badge is based on a magnetic field emitted from a constant power from a voltage regulation of the antenna device.

Such a solution has the disadvantage of being difficult to implement. Indeed, the antenna device being supplied with voltage by the vehicle battery voltage, at each variation of this voltage (due to different actions such as a vehicle start, or an engine stop, etc.), it is necessary to readjust to allow the antenna device to transmit at a constant power.

We also know the document FR-A-2,834,344 which discloses a method of detecting a badge based on a measurement of the magnetic field. This method does not take into account the impedance variations that occur during use of the antenna device in the vehicle.

We finally know the document EP-A-1,189,302 which discloses a method of calibrating an antenna for detecting a badge when mounting this antenna on the vehicle.

Object of the invention

The object of the invention is therefore more particularly to enable detection of an identifying object with a simpler solution.

It therefore proposes a method of detecting an identification object in an area around an antenna device (s), according to the features of claim 1.

Thus, as will be seen in detail below, there will be a detection of an identifying object through the calibration of the antenna device (s) and a comparison with a nominal magnetic field without the need for voltage regulation.

• Le signal de calibrage est non intelligible par l'objet d'identification. Cela permet à l'objet d'identification de recevoir rapidement le signal fonctionnel par la suite.
• Le procédé comporte une étape supplémentaire selon laquelle :
• lors de l'envoi du signal de calibrage, on mesure un courant circulant dans le dispositif d'antenne(s), et
• on compare le courant mesuré à un courant initial de sorte à déterminer la puissance de réglage.

According to non-limiting embodiments, the method according to the invention has the following additional features.

• The calibration signal is not intelligible by the identification object. This allows the identification object to quickly receive the functional signal thereafter.
• The method comprises a further step that:
• when sending the calibration signal, a current flowing in the antenna device (s) is measured, and
• the measured current is compared to an initial current so as to determine the adjustment power.

• Une puissance est réglée avec une tension de rapport cyclique donné. Cette régulation est simple à mettre en oeuvre. De plus, cela permet de compenser les variations de la tension d'alimentation du dispositif d'antenne(s).
• La tension est un signal symétrique. Cela permet de supprimer les harmoniques de rang pair dans le signal courant mesuré et d'obtenir ainsi une mesure plus précise du courant.
• Le rapport cyclique est égal à 1/3. Cela permet de supprimer les harmoniques de rang multiples de trois dans le signal courant mesuré et d'obtenir ainsi une mesure plus précise du courant.
• La tension est générée au moyen d'un étage de puissance avec commande à pont complet ou à demi-pont. Cela permet d'obtenir une plus grande gamme de courants.
• Le signal de calibrage est déclenché en fonction d'un événement particulier. Cela permet d'avoir régulièrement un signal de calibrage réactualisé et donc par la suite une mesure régulière et précise du champ magnétique émis par le dispositif d'antenne(s).
• Selon une première variante, l'événement particulier est un accès véhicule. Cela permet de prendre en compte les variations du champ magnétique émis dues à des événements extérieurs tel que des variations de température.
• Selon une deuxième variante, l'événement particulier est une variation de tension batterie. Cela permet de prendre en compte ces variations dans le calibrage.
• Le procédé comporte une étape initiale supplémentaire d'écriture d'une valeur de seuil fixe dans l'objet d'identification. Cela permet d'obtenir un champ fixe de réception à partir duquel l'objet d'identification peut recevoir des signaux du dispositif d'antenne(s) et communiquer avec un dispositif de commande associé.
• La valeur de seuil fixe est fonction d'un champ magnétique nominal. Cela permet à l'objet d'identification de répondre à un signal émis du dispositif d'antenne(s) lorsque il est dans la zone correspondant au champ magnétique nominal..
• La zone autour du dispositif d'antenne(s) est définie par le champ magnétique nominal.
• La zone autour du dispositif d'antenne(s) correspond à un habitacle de véhicule. Ainsi, on détermine si l'objet d'identification se situe dans l'habitacle du véhicule pour autoriser un démarrage du véhicule.

Thus, the calibration signal depends on a current measurement that is simple to implement.

• A power is set with a given duty cycle voltage. This regulation is simple to implement. In addition, this makes it possible to compensate for variations in the supply voltage of the antenna device (s).
• The voltage is a symmetrical signal. This makes it possible to suppress the even-numbered harmonics in the measured current signal and thus obtain a more accurate measurement of the current.
• The duty ratio is 1/3. This makes it possible to suppress the multiple harmonics of three in the measured current signal and thus obtain a more accurate measurement of the current.
• The voltage is generated by means of a power stage with full-bridge or half-bridge control. This allows for a greater range of currents.
• The calibration signal is triggered according to a particular event. This makes it possible to regularly have an updated calibration signal and therefore subsequently a regular and accurate measurement of the magnetic field emitted by the antenna device (s).
• According to a first variant, the particular event is a vehicle access. This makes it possible to take into account the variations in the magnetic field emitted due to external events such as temperature variations.
• According to a second variant, the particular event is a battery voltage variation. This makes it possible to take into account these variations in the calibration.
• The method includes an additional initial step of writing a fixed threshold value in the identification object. This makes it possible to obtain a fixed reception field from which the identification object can receive signals from the antenna device (s) and communicate with an associated control device.
• The fixed threshold value is a function of a nominal magnetic field. This allows the identification object to respond to a signal transmitted from the antenna device (s) when it is in the area corresponding to the nominal magnetic field.
• The area around the antenna device (s) is defined by the nominal magnetic field.
• The area around the antenna device (s) corresponds to a vehicle interior. Thus, it is determined whether the identification object is in the passenger compartment of the vehicle to allow a start of the vehicle.

• le dispositif de commande est apte à :
  • émettre un signal de calibrage en direction du dispositif d'antenne(s) pour déterminer une puissance de réglage,
  • émettre un signal fonctionnel en direction du dispositif d'antenne(s) correspondant à la puissance de réglage de sorte que le dispositif d'antenne(s) émette un champ magnétique déterminé,
  • déterminer si l'objet d'identification se trouve dans la zone autour du dispositif d'antenne(s) en fonction d'une comparaison effectuée entre le champ magnétique reçu par l'objet d'identification et un champ magnétique nominal,
• l'objet d'identification est apte à mesurer le champ magnétique reçu correspondant au champ magnétique émis et à le comparer avec le champ magnétique nominal.

According to a second object, the invention relates to a system for detecting an identification object in an area around an antenna device (s), comprising a control device, an antenna device (s) and an identification object, characterized in that:

• the control device is able to:
  • transmit a calibration signal to the antenna device (s) to determine a setting power,
  • transmitting a functional signal to the antenna device (s) corresponding to the adjustment power so that the antenna device (s) emits a determined magnetic field,
  • determining whether the identification object is in the area around the antenna device (s) based on a comparison made between the magnetic field received by the identification object and a nominal magnetic field,
• the identification object is able to measure the received magnetic field corresponding to the emitted magnetic field and to compare it with the nominal magnetic field.

• recevoir un signal de calibrage correspondant à une puissance initiale déterminée,
• recevoir un signal fonctionnel correspondant à une puissance de réglage déterminée de sorte à émettre un champ magnétique déterminé, et
• transmettre le signal fonctionnel à l'objet d'identification, ce dernier recevant un champ magnétique fonction du champ magnétique émis par le dispositif d'antenne(s).

According to a third object, the invention relates to an antenna device (s) adapted to cooperate with an identification object, characterized in that it is able to:

• receive a calibration signal corresponding to a determined initial power,
• receiving a functional signal corresponding to a determined adjustment power so as to emit a determined magnetic field, and
• transmitting the functional signal to the identification object, the latter receiving a magnetic field depending on the magnetic field emitted by the antenna device (s).

• émettre un signal de calibrage en direction du dispositif d'antenne(s) pour déterminer une puissance de réglage, et
• émettre un signal fonctionnel en direction du dispositif d'antenne(s) correspondant à la puissance de réglage déterminée de sorte que le dispositif d'antenne(s) émette un champ magnétique déterminé.

According to a fourth object, the invention relates to a control device adapted to cooperate with an antenna device (s) and with an identification object, characterized in that it comprises a signal transmitter for:

• transmitting a calibration signal to the antenna device (s) to determine a setting power, and
• transmitting a functional signal in the direction of the antenna device (s) corresponding to the setting power determined so that the antenna device (s) emits a determined magnetic field.

According to a non-limiting embodiment, the control device further comprises a signal receiver for receiving a response from the identification object according to a comparison made between a received magnetic field and a nominal magnetic field.

According to a non-limiting embodiment, the comparison is performed by the identification object.

According to a fifth object, the invention relates to a motor vehicle comprising a passenger compartment in which is arranged a control device according to any one of the preceding characteristics, an antenna device (s) characterized according to any one of the preceding characteristics, the two devices being able to cooperate with an identification object.

Other features and advantages of the present invention will be better understood with the aid of the description of non-limiting examples.

Detailed description of non-limiting embodiments of the invention

A vehicle V equipped with a signal transmission-reception device DER for controlling an antenna device A, and the antenna device A comprises, in a nonlimiting example, a plurality of antennas, here antennas called external AX and so-called indoor antennas AI, all these antennas cooperating with a receiver-transmitter ID, all forming a detection system described below.

In the nonlimiting example, there are five external antennas AX four AX1, AX2, AX3 and AX4 are located outside the cabin VH vehicle V, here on the door handles, and an AX5 in the bumper rear hitch VC of the vehicle. In addition two indoor antennas AI1, AI2 are located in the passenger compartment VH, here at the front and rear of the vehicle. Each antenna is powered by low-frequency alternating current by the DER transceiver device and emits a Be magnetic field, named BeI for indoor antennas and BeX for external antennas.

By means of their respective emitted magnetic field B, the external antennas AX make it possible to detect whether the receiver-transmitter ID is located near the vehicle V, in a nonlimiting example at a distance of less than 1.5 mm, whereas the antennas AI interiors detect whether the receiver-transmitter ID is in the passenger compartment VH of the vehicle.

The receiver-transmitter ID, in this application, is in a non-limiting example, an identification object ID carried by a user of the vehicle V, for example a badge, a key, a keyfob called "keyfob" etc. The example of the identification badge will be taken as an example in the following description.

Using the alternating current, the antennas A communicate with the ID badge by transmitting data by emitting a low frequency signal BF and the ID badge responds by emitting an RF radio frequency signal. In a non-limiting example, the low frequency signal BF is around 125 kHz and the radio frequency RF signal is around 433 MHz. We can go down to 20kHz for the low frequency signal BF or go to GigaHz for the RF radio frequency signal according to the frequency bands available for different countries (315MHz for Asia, 868Mhz for some European countries or 915MHz in America etc.).

Depending on the response, the antennas A determine whether the badge ID is allowed to open the doors of the vehicle, or whether it is allowed to start the vehicle. Note that in a non-limiting example, for the badge ID is allowed to open the doors, the user must for example touch a doorknob. For this purpose, the handles include appropriate detectors.

The external antennas AX make it possible to determine a first zone of communication with the badge ID to authorize a vehicle access. This zone is defined by the magnetic field emitted by said antennas. External AX antennas must therefore guarantee at least a minimum distance from which the ID badge is authorized to access the vehicle.

The internal antennas AI make it possible to determine a second zone ZO for communication with the badge ID to authorize a start. This zone is defined by the magnetic field emitted by the internal antennas. The internal antennas AI must therefore guarantee a fixed zone from which the ID badge is authorized to start the vehicle, this zone being the passenger compartment VH of the vehicle V.

Note that in practice, the magnetic field emitted by these indoor antennas AI has a greater coverage than the passenger compartment VH but is limited by the metal carcass of the passenger compartment VH of the vehicle V and overflows through the openings of the windows.

The detection of the ID badge in the second zone ZO is based on the fact that the badge is initialized with a fixed threshold value S0 according to a magnetic field received B0 called nominal field. Without limitation, this fixed threshold value S0 is a power P0 corresponding to the nominal field B0.

It will be recalled that a magnetic field received Br by the badge ID is a function of the emitted magnetic field Be of the antenna device A, the latter defining the zone ZO around it representative of its magnetic field Be, which has also been called a zone Communication.

It is possible to analyze the position of the badge ID with respect to an antenna A of the antenna device as a function of the magnetic field Be of this antenna A and therefore of the corresponding magnetic field Br received.

It can be seen that the further the ID badge is located far from an antenna A emitting a magnetic field emitted Be, the lower the magnetic field received Br corresponding is weak. When the ID badge is at the same location as the antenna A, the received magnetic field Br is theoretically equal to the emitted magnetic field Be.

The nominal magnetic field B0 therefore corresponds to a nominal communication zone ZO in which an ID badge can communicate with an antenna A and the transmission-reception device DER.

When the ID badge is outside this ZO area (the received magnetic field Br is smaller than the nominal received magnetic field B0), the ID badge does not respond to the signals sent by the antenna device A or sends an RF response radio voluntarily wrong. This means that it is located outside the passenger compartment VH of the vehicle. In the opposite case, it responds by emitting an RF radio frequency signal. It should be noted that this nominal magnetic field B0 is set so as to avoid the parasitic magnetic fields Bb originating from the radio disturbances as illustrated in FIG. 2 and its value is greater than the value of the parasitic magnetic fields.

In one example, the ID badge is positioned relative to an internal antenna AI1.

The antenna AI1 emits a magnetic field Be at a power of 15 Watts. The fixed threshold value S0 is set at P0 = 5 Watts. The ID badge, when in the ID1 position, receives a magnetic field Br1 whose power P1 is 4.5 Watts and therefore less than the fixed threshold S0; so it's outside of the zone ZO defined by the threshold S0 will therefore not communicate with the antenna AI1 and the transmission-reception device DER.

The ID badge, when in the ID2 position, receives a magnetic field Br2 whose power P2 is 5.5 Watts and therefore greater than the fixed threshold S0; it is therefore in the ZO communication zone and will therefore communicate with the antenna AI1 and the transmission-reception device DER.

Thus, the method of detecting the badge ID to know if it is in the communication zone ZO of an antenna, in particular an indoor antenna AI is carried out as follows.

In an initial step 0), during the manufacture of the ID badge, the fixed threshold value S0 is written in a memory of the ID tag, for example a rewritable type EEPROM memory.

Then, when using the ID badge and the antenna device A, that is to say in operating mode,
In a first step 1), a calibration signal S_CAL, also called a calibration frame, is sent in the direction of the antenna device A at an initial power PI determined to determine an adjustment power PR for the antenna device A.

In a non-limiting embodiment, the calibration signal S_CAL is triggered according to a particular event.

In a non-limiting example, the particular event is a vehicle access. Indeed, a vehicle access is representative of a change in the environment of the antenna device A, a variation of temperatures (due for example to different seasons), which influences the antenna device components and consequently causes a variation of its impedance Z and thus of its magnetic field emitted Be.

In another nonlimiting example, the particular event is a variation of supply voltage, here the battery voltage Ubat (which may vary after an engine start or engine stop for example). This makes it possible to take into account these variations in the calibration, these variations affecting the current flowing in the antenna device. These variations of battery voltages are thus compensated, since in this case the current Irm is measured after a battery voltage variation.

The purpose of this step is therefore to take into account the variations of the impedance Z of the antenna device A in the determination of the magnetic field emitted by the antenna device A and consequently in the determination of a power of PR setting to be applied to the antenna device A.

This takes into account the impedance variations Z that appear during the use of the antenna device (s) in the vehicle, so during its operation.

It will be noted that the calibration signal S_CAL is unintelligible by the identification object ID. Thus, although the identification object ID receives it, it does not respond to this signal S_CAL. This avoids the badge an additional activation-deactivation period to receive this signal. Thus, the ID badge will receive faster data included in the S_FONC functional signal that it will receive thereafter.

Thus, in a first substep 1a), an initial power PI is determined to send the calibration signal S_CAL.

The initial power PI is obtained by means of a calibration voltage UC of initial duty cycle α1 corresponding to a theoretical initial current Ith. It will be noted that this theoretical initial current Ith is determined experimentally by vehicle tests. Its value varies according to the type of vehicle V.

In a non-limiting embodiment, this voltage is square. This avoids energy dissipation in the power stage for applying the voltage described below. Indeed, there is a heat energy consumption only during transitions phases unlike a typical sinusoidal voltage where the consumption is significantly higher .. This power stage does not heat too much.

In a second substep 1b) , when sending the calibration signal S_CAL, the actual current Irm flowing in the antenna device A is measured. In fact, although an initial power PI corresponding to theoretical initial current Ith, because of the impedance Zr of the antenna device A, the current flowing in said device is in practice not equal to the theoretical initial current Ith.

The measurement of the current Irm flowing in the antenna device A can be carried out in a simple manner by a peak amplitude detector C described below. This current Irm is an alternating current whose frequency spectrum comprises harmonics h.

In a nonlimiting embodiment, the antenna device A is tuned to the transmission frequency (the frequency being for example 125 kHz). This makes it possible to emit a larger magnetic field in amplitude at the transmission frequency, and to have a bandpass filter FL. The bandpass filter FL thus makes it possible to reduce the amplitude of the harmonics h (except for the harmonic of rank 1).

Indeed, on transmission, on the side of the antenna device A, the value of the current Irm flowing in the antenna device A is equal to the sum of the harmonics h which are present in the passband of the filter included in the antenna device A. Depending on the selectivity of the filter, we will have all the harmonics if the filter is broad band, or only part of the harmonics if the filter is narrow band. At the emission therefore, the value of the emitted field Be is a function of this current Irm with harmonics h.

At the reception, on the ID badge side, the value of the current Irm which is taken into account is equal only to the harmonic h1 of rank 1 called fundamental. Indeed, the received magnetic field Br (and therefore the fixed threshold value S0) corresponds to the magnetic field emitted Be at the fundamental value only and not at the sum of the harmonics.

It is therefore necessary to precisely determine the power emitted on the harmonic h1 of rank 1 to allow to emit a transmitted magnetic field Be corresponding precisely to the nominal field B0 and thus to the fixed threshold S0 of the ID badge. It is therefore necessary to measure the current Irm so as to eliminate as much as possible the harmonics other than the fundamental h1.

This is done by means of the initial cycle duty UC calibration voltage α1 which makes it possible to obtain the initial power PI.

When the calibration voltage UC is any square signal, all the harmonics of the measured current Irm may be present.

In a first non-limiting embodiment, the calibration voltage UC is a symmetrical voltage. In this case, the even-order harmonics of the measured current Irm have been suppressed. As can be seen, the voltage UC is symmetrical with respect to the point PT. The symmetrical CPU voltage will make it possible to obtain precise generation and precise measurement of the corresponding initial power PI on the harmonic h1 of rank 1 by suppressing parasitic currents due to other harmonics.

Indeed, during a frequency representation, a harmonic of rank n is represented by the term a _n cosnωt + b _n sinnωt.

The voltage UC is an odd function, ie f (-x) = -f (x), its development in Fourier series therefore only includes sine terms, the coefficients a _n being zero.

on obtient Cn=j(2E/πn)sin(nπα1).sin(n(π/2))
et $$\mathrm{bn}\mathrm{=}\left(\mathrm{4}\mathrm{E}\mathrm{/}\mathrm{\pi n}\right)\mathrm{.}\sin \left(\mathrm{n\pi \alpha }\mathrm{1}\right)\mathrm{.}\sin \left(\mathrm{n}\left(\mathrm{\pi }\mathrm{/}\mathrm{2}\right)\right)$$

avec ω = 2π/T, avec T la période et E l'amplitude de la tension d'alimentation Ubat du dispositif d'antennes.So, knowing that $\mathrm{Cn}=\frac{1}{T}\int \mathrm{f}\left(\mathrm{x}\right){\mathrm{e}}^{\mathrm{-}\mathrm{jn\omega x}}\mathrm{dx\; and\; Cn}\mathrm{=}\frac{\mathrm{1}}{\mathrm{2}}\left(\mathrm{year}\mathrm{-}\mathrm{JBN}\right)$

we obtain Cn = j (2E / πn) sin (nπα1) .sin (n (π / 2))
and $$\mathrm{bn}\mathrm{=}\left(\mathrm{4}\mathrm{E}\mathrm{/}\mathrm{\pi n}\right)\mathrm{.}\sin \left(\mathrm{n\pi \alpha }\mathrm{1}\right)\mathrm{.}\sin \left(\mathrm{not}\left(\mathrm{\pi }\mathrm{/}\mathrm{2}\right)\right)$$

with ω = 2π / T, with T the period and E the amplitude of the supply voltage Ubat of the antenna device.

soit $$\mathrm{f}\left(\mathrm{x}\right)\mathrm{=}\mathrm{\Sigma }\left(\mathrm{4}\mathrm{E}\mathrm{/}\left(\mathrm{\pi }\left(\mathrm{2}\mathrm{p}\mathrm{+}\mathrm{1}\right)\right)\right)\mathrm{.}\sin \left(\left(\mathrm{2}\mathrm{p}\mathrm{+}\mathrm{1}\right)\mathrm{\pi \alpha }\mathrm{1}\right)\mathrm{.}\sin \left(\left(\mathrm{2}\mathrm{p}\mathrm{+}\mathrm{1}\right)\left(\mathrm{\pi }\mathrm{/}\mathrm{2}\right)\right)\mathrm{.}\sin \left(\mathrm{2}\mathrm{p}\mathrm{+}\mathrm{1}\right)\mathrm{\omega x}\mathrm{,}$$

avec $$\mathrm{p}\mathrm{=}\mathrm{0}\mathrm{,}\mathrm{\dots }\mathrm{,}\mathrm{\infty }$$

ce qui donne le spectre avec les harmoniques à la Figure 7.The Fourier series corresponding to the symmetrical voltage signal UC is therefore equal to: $$\mathrm{f}\left(\mathrm{x}\right)\mathrm{=}\mathrm{\Sigma }\left(\mathrm{4}\mathrm{E}\mathrm{/}\mathrm{\pi n}\right)\mathrm{.}\sin \left(\mathrm{n\pi \alpha }\mathrm{1}\right)\mathrm{.}\sin \left(\mathrm{not}\left(\mathrm{\pi }\mathrm{/}\mathrm{2}\right)\right)\mathrm{.}\mathrm{sin\; n\omega x}\mathrm{,}\mathrm{with\; n}\mathrm{=}\mathrm{1}\mathrm{,}\mathrm{...}\mathrm{,}\mathrm{\infty }$$

is $$\mathrm{f}\left(\mathrm{x}\right)\mathrm{=}\mathrm{\Sigma }\left(\mathrm{4}\mathrm{E}\mathrm{/}\left(\mathrm{\pi }\left(\mathrm{2}\mathrm{p}\mathrm{+}\mathrm{1}\right)\right)\right)\mathrm{.}\sin \left(\left(\mathrm{2}\mathrm{p}\mathrm{+}\mathrm{1}\right)\mathrm{\pi \alpha }\mathrm{1}\right)\mathrm{.}\sin \left(\left(\mathrm{2}\mathrm{p}\mathrm{+}\mathrm{1}\right)\left(\mathrm{\pi }\mathrm{/}\mathrm{2}\right)\right)\mathrm{.}\sin \left(\mathrm{2}\mathrm{p}\mathrm{+}\mathrm{1}\right)\mathrm{\omega x}\mathrm{,}$$

with $$\mathrm{p}\mathrm{=}\mathrm{0}\mathrm{,}\mathrm{...}\mathrm{,}\mathrm{\infty }$$

which gives the spectrum with the harmonics in Figure 7.

The value of the fundamental h1 is given by: $$\mathrm{h}\mathrm{1}\mathrm{=}\left(\mathrm{4}\mathrm{E}\mathrm{/}\mathrm{\pi }\right)\mathrm{.}\mathrm{sin\; \pi \alpha }\mathrm{1.}\mathrm{sin\; \omega x}$$

Moreover, it will be noted that the fact of having a square voltage avoids energy dissipation in the transistors of the power stage P. In fact, there is a heat energy consumption only during the transition phases, unlike a typical sinusoidal voltage where the consumption is significantly higher. This power stage P does not heat too much.

It will be noted that the value of the adjustable duty cycle α1 makes it possible to adjust the value of the initial power PI.

Thus, the symmetrical square voltage makes it possible, on the one hand, to adjust the initial power transmitted PI to a desired value corresponding to the desired communication zone ZO (and thus to accurately generate the transmitted power PI) and on the other hand to obtain an accurate measurement of the actual transmitted power PI corresponding to the effective received power of the ID badge because the harmonics of even rank are suppressed.

In a second nonlimiting embodiment, the calibration voltage UC comprises a duty cycle of 1/3 which corresponds to an offset of π / 3 of the voltage signal UC. As can be seen in Figure 8, in this case the multiple rank harmonics 3 of the measured current Irm have been suppressed.

Note that the two embodiments can be combined. In this case, only the harmonics of rank 1 and 5 remain, the latter being negligible.

Thus, a precise measurement of the current Irm flowing in the antenna device A is obtained. The measured current Irm is therefore in this case representative of the amplitude of the fundamental of the emitted magnetic field.

Therefore, it is possible to deduce the initial power transmitted PI (and thus the emitted magnetic field Be) by the antenna device A corresponding precisely to the received power Pr, knowing that the emitted magnetic field Be is proportional to the measured current Irm.

$${\mathrm{B}}_{\mathrm{\theta }}\mathrm{=}\left(\mathrm{Ae\; Im}\mathrm{/}\mathrm{4}\mathrm{\pi }{\mathrm{d}}^{\mathrm{3}}\right)\mathrm{*}\mathrm{sin\; \theta }\mathrm{,}$$

et $$\mathrm{B\varphi }=0.$$

avec Ae la surface effective d'une antenne par laquelle s'écoule le champ magnétique B, d la distance qui permet une mesure du champ magnétique B à partir du centre de l'antenne.It will be recalled that, in a manner known to those skilled in the art, a magnetic field B comprises three components in an orthogonal space x, y, z as illustrated in FIG. 10 which are as follows. $${\mathrm{B}}_{\mathrm{\mu }}\mathrm{=}\left(\mathrm{Ae\; Im}\mathrm{/}\mathrm{2}\mathrm{\pi }{\mathrm{d}}^{\mathrm{3}}\right)\mathrm{*}\mathrm{cos\; \theta }\mathrm{,}$$

$${\mathrm{B}}_{\mathrm{\theta }}\mathrm{=}\left(\mathrm{Ae\; Im}\mathrm{/}\mathrm{4}\mathrm{\pi }{\mathrm{d}}^{\mathrm{3}}\right)\mathrm{*}\mathrm{sin\; \theta }\mathrm{,}$$

and $$\mathrm{B\phi }=0.$$

with Ae the effective surface of an antenna through which the magnetic field B, d the distance which allows a measurement of the magnetic field B from the center of the antenna.

It is also recalled that Ae = N _w * A * μ _rod with N _w the number of turns in the antenna, A the cross section of the ferrite of the turns, and μ _rod the apparent permeability of the ferrite.

It should be noted that the calibration voltage UC can be obtained by means of an H-bridge power stage P with full bridge control described below.

In a third substep 1c) , the measured current Irm is compared with the theoretical initial current Ith. The difference will make it possible to determine the real impedance Zr of the antenna device A and consequently to determine the adjustment power PR to be applied to the antenna device A.

It will be recalled that to obtain a zone ZO around the antenna device A corresponding to the passenger compartment of the vehicle VH, a transmitted field Be is determined corresponding to a power PR. This field that we want to obtain whose value is known therefore corresponds to a known current Iv. To supply the antenna device A with this known and desired current Iv, it is necessary to adjust the power PR of said antenna device taking into account its impedance Zr.

The adjustment power PR is adjusted by means of a functional voltage UF of adjustment duty cycle α2.

In a fourth substep 1d) , a duty cycle α2 is thus determined.

$$\mathrm{=}\sin \left(\mathrm{\alpha }\mathrm{1}\mathrm{\pi }\right)\mathrm{*}\left(\mathrm{Iv}\mathrm{/}\mathrm{Irm}\right)$$

d'où $$\mathrm{\alpha }\mathrm{2}\mathrm{=}\left(\mathrm{1}\mathrm{/}\mathrm{\pi }\right)\mathrm{*}\mathrm{Arcsin}\left(\sin \left(\mathrm{\alpha }\mathrm{1}\mathrm{\pi }\right)\mathrm{*}\left(\mathrm{Iv}\mathrm{/}\mathrm{Im}\right)\right)$$

The calculation of the duty ratio α2 is deduced in the following manner. We have Irm = (Ubat * sin (α1π)) / Zr, where Zr = (Ubat * sin (α1π)) / Irm
and Iv = (Ubat sin (α2π)) / Zr
from where $$\sin \left(\mathrm{\alpha }\mathrm{2}\mathrm{\pi }\right)\mathrm{=}\left(\mathrm{Zr}\mathrm{*}\mathrm{iv}\right)\mathrm{/}\mathrm{Ubat}\mathrm{=}\left(\mathrm{Ubat}\mathrm{*}\sin \left(\mathrm{\alpha }\mathrm{1}\mathrm{\pi }\right)\mathrm{*}\mathrm{iv}\right)\mathrm{/}\left(\mathrm{MRI}\mathrm{*}\mathrm{Ubat}\right)$$

$$\mathrm{=}\sin \left(\mathrm{\alpha }\mathrm{1}\mathrm{\pi }\right)\mathrm{*}\left(\mathrm{iv}\mathrm{/}\mathrm{MRI}\right)$$

from where $$\mathrm{\alpha }\mathrm{2}\mathrm{=}\left(\mathrm{1}\mathrm{/}\mathrm{\pi }\right)\mathrm{*}\arcsin \left(\sin \left(\mathrm{\alpha }\mathrm{1}\mathrm{\pi }\right)\mathrm{*}\left(\mathrm{iv}\mathrm{/}\mathrm{im}\right)\right)$$

It may be noted that in practice, in the calculation [1] of the duty ratio α2, the actual impedance Zr of the antenna device A no longer intervenes.

In a first nonlimiting embodiment, the adjustment duty ratio α2 is calculated by means of a microprocessor of the transmission-reception device DER described below.

In a second simpler and faster embodiment, the duty cycle α2 is defined based on a pre-filled correspondence table (not shown) using the formula sin (α1π) / sin (α2π) = Irm / Iv [2] . In this table, at each difference between the measured current Irm flowing in the antenna device during the sending of a calibration signal S_CAL and the theoretical current Ith, the cyclic adjustment ratio α2 corresponding to a desired functional current Iv which takes into account the variations of the impedance Zr of the antenna device A.

Such a table is as follows: Irm1 Irm2 Irm3 Etc ... Iv1 α2 ₁₁ α2 ₁₂ α2 ₁₃ α2 _{1 ...} Iv2 α2 ₂₁ α2 ₂₂ α2 ₂₃ α2 _{2 ...} IV3 α2 ₃₁ α2 ₃₂ α2 ₃₃ α2 _{3 ...} Etc ... Etc ... Etc ... Etc ... Etc ...

With Iv1, Iv2, Iv3, etc., the different values of the desired current Iv, Irml, Irm2, Irm3, etc., the different values of the measured current Irm flowing in the antenna device A and α21 ₁₁ ... α2 ₂₁ ... α2 ₃₁ etc ... the different corresponding duty cycle values.

The application of the formula [1] or again The choice of the adjustment duty cycle α2 in this correspondence table is equivalent to using a curve CZ representative of the actual impedance Zr to determine the duty cycle α2 to be applied to the device. antennas A, as illustrated in the diagram of Figure 11 in a non-limiting example.

The abscissa is represented by the cyclic ratio α and the ordinate by the current I. As can be seen, for a theoretical initial current Ith, an initial duty cycle voltage α1 is applied. The correspondence between this initial cyclic ratio α1 and the theoretical initial current Ith lies on a curve CZth representative of the theoretical impedance Zth of the antenna device A. The actual current Irm circulating in the device is measured at this initial duty cycle α1 . The correspondence between this cyclic ratio α1 and the actual measured current Irm is on a curve representative of the real impedance Zr of the antenna device A. Two maximum curves CZrmax and minimum CZrmin of this real impedance Zr are represented. In the example, we took the curve CZmax corresponding to the maximum of the real impedance Zrmax. Finally, for the desired current Iv, the corresponding adjustment cyclic ratio α2 is determined by taking the curve of the real impedance Zr, here CZrmax and making a projection on the abscissa axis.

Thus the duty cycle α2 was found to apply the desired functional voltage UF to obtain a desired power PR in the antenna device A.

It will be noted that the functional voltage UF can be obtained by means of an H-bridge power stage P with full-bridge and half-bridge control described below.

This first calibration step 1) therefore corresponds to a self-calibration of the SYS detection system to determine the adjustment power PR. Indeed, no external measuring device is necessary for this calibration. Moreover, this auto-calibration is dynamic because it is launched during the operation of the antenna device (s), and not during its development of the antenna device (s) in the factory for example.

In a second step 2), after sending the calibration signal S_CAL, a functional signal S_FONC, also called a functional frame, is thus sent in the direction of the antenna device A, as shown in FIG. above so that the antenna device A emits a determined magnetic field Be corresponding to the desired area ZO and more particularly to the passenger vehicle VH in the example taken from the vehicle application.

In a third step 3), the received magnetic field Br is measured by the identification object ID corresponding to the transmitted magnetic field Be of the antenna device A. This measurement is carried out by means of a measurement device of the type RSSI d an amplifier ("Received Signal Strength Indication") well known to those skilled in the art included in the identification object ID.

In a fourth step 4), the magnetic field received Br is compared with the nominal magnetic field B0. This comparison is performed in the identification object ID.

In a fifth step 5), it is determined whether the ID badge is in the zone ZO around the antenna device A according to this comparison.

Thus, the ID badge is in the zone ZO around the antenna device A, and therefore inside the passenger compartment VH vehicle, if the magnetic field received Br is greater than the nominal magnetic field B0. The badge ID therefore returns an affirmative response REPOK to a DC control device of the transmission-reception device DER. The latter therefore allows a vehicle start for example.

On the other hand, the badge ID is located outside the zone ZO, and therefore outside the cockpit VH vehicle, if the magnetic field received Br is less than nominal magnetic field B0. In this case, either the ID badge returns no response, it acts as if it has not received the S_FONC functional signal of the antenna device A, or it sends a negative REPNOK response to the DC control device, as illustrated. in Figure 5. The latter therefore prohibits any vehicle start for example.

In another variant, whether inside or outside the zone ZO, the badge ID systematically sends a response REP including the result of the comparison.

• un dispositif d'émission-réception DER comprenant :
  • un dispositif de commande DC,
  • un étage de puissance P,
  • un dispositif de mesure de courant C,
  • un récepteur RE de signaux pour notamment recevoir une réponse REPOK, REPNOK du badge d'identification ID en fonction de la comparaison effectuée entre le champ magnétique reçu Br et le champ magnétique nominal B0.
• le dispositif d'antennes A, et
• un récepteur-émetteur, ici, le badge d'identification ID.

The method which has been described is implemented by a detailed SYS detection system in a non-limiting embodiment and comprising

• a DER transceiver device comprising:
  • a DC control device,
  • a power stage P,
  • a current measuring device C,
  • a signal receiver RE, in particular for receiving a REPOK, REPNOK response of the identification badge ID as a function of the comparison made between the received magnetic field Br and the nominal magnetic field B0.
• the antenna device A, and
• a receiver-transmitter, here, the identification badge ID.

Note that according to a non-limiting embodiment, all the elements of the transceiver device DER are on the same electronic card. This allows a faster and more reliable dialogue between these different elements. On the contrary, when these elements are separated, the communication links connecting them can be more easily disturbed and the flow rates of these links can be lower.

The identification badge ID is known to those skilled in the art, it is not described here.

The other elements are described in more detail below.

• The antenna device A.

In a first nonlimiting embodiment, the antenna device A is composed of a circuit RL. The latter requires amplifying the supply voltage of the antenna device to allow appropriate magnetic field emission.

In a second nonlimiting embodiment, the antenna device A is composed of an RLC circuit. The latter makes it possible from the supply voltage of the antenna device A, which is here the battery voltage Ubat of the vehicle V, to directly amplify the current I flowing in the antenna device A to enable a field emission. magnetic field, without using voltage control unlike the first embodiment. It is therefore a simpler solution to implement to obtain amplification. This RLC circuit also acts as a bandpass filter as seen previously.

• The DC control device includes:

• an emitter EM of signals for including:
  • transmit the calibration signals S_CAL towards the antenna device A,
  • transmit the functional signals S_FONC towards the antenna device A,
  • transmit control signals to the power stage P to supply the supply voltage Ubat to the antenna device A,
• a CMP comparator of current Inn, Ith, and
• a calculation member CALC (for example a microprocessor or an ASIC) making it possible in particular to adapt the cyclic ratios α1 and α2 of the UC and functional calibration voltages UF.

• le récepteur RE de signaux pour notamment recevoir une réponse REPOK, REPNOK du badge d'identification ID en fonction de la comparaison effectuée entre le champ magnétique reçu Br et le champ magnétique nominal B0.

In a non-limiting embodiment, the control device DC may further comprise:

• the signal receiver RE, in particular for receiving a response REPOK, REPNOK of the identification badge ID as a function of the comparison made between the received magnetic field Br and the nominal magnetic field B0.

• The P power stage.

It provides the calibration voltage UC for adjusting the initial power PI and the functional voltage UF for adjusting the setting power PR of the antenna device A.

In a non-limiting embodiment, the power stage P is H-bridge with full-bridge or half-bridge control. It is illustrated in Figure 13. It comprises in particular four switches S1 to S4. These switches are in a non-limiting example of the MOSFET transistors.

• Entre les intervalles t0-t1 et t2-t3, soit tous les interrupteurs sont ouverts, soit les interrupteurs S2 et S4 sont fermés, soit les interrupteurs S1 et S3 sont fermés, les autres étant ouverts. La tension UC est nulle.
• Entre l'intervalle t1-t2, les interrupteurs S1-S4 sont fermés, les autres étant ouverts. La tension UC est positive.
• Entre l'intervalle t3-t4, les interrupteurs S2-S3 sont fermés, les autres étant ouverts. La tension UC est négative.

In order to provide a voltage, the power stage P operates as a full bridge in the following manner. The example is taken for symmetrical voltage

• Between the intervals t0-t1 and t2-t3, either all the switches are open, the switches S2 and S4 are closed, or the switches S1 and S3 are closed, the others being open. The UC voltage is zero.
• Between the interval t1-t2, the switches S1-S4 are closed, the others being open. The UC voltage is positive.
• Between the interval t3-t4, the switches S2-S3 are closed, the others being open. The UC voltage is negative.

The two diagonals of the bridge are controlled by two control signals delayed relative to each other by half a period thus making it possible to obtain symmetry.

When the power stage P operates in full bridge, the obtained voltage makes it possible to obtain a first range of currents G1 = [I11-I12].

• Entre les intervalles t0-t1 et t2-t3, soit les trois autres interrupteurs S1-S2-S3 sont ouverts, soit l'interrupteur S2 est fermé, les deux autres S1-S3 étant ouverts. La tension UC/UF est nulle.
• Entre l'intervalle t1-t2, l'interrupteur S1 est fermé, les deux autres S2-S3 étant ouverts. La tension UC/UF est positive. Ou l'interrupteur S2 est fermé, les deux autres S1-S3 étant ouverts. La tension UC/UF est négative.

The power stage P operates in the following half-bridge manner. It will be noted that the switch S4 is always closed.

• Between the intervals t0-t1 and t2-t3, the other three switches S1-S2-S3 are open, or the switch S2 is closed, the other two S1-S3 being open. UC / UF voltage is zero.
• Between the interval t1-t2, the switch S1 is closed, the other two S2-S3 being open. The UC / UF voltage is positive. Or the switch S2 is closed, the other two S1-S3 being open. UC / UF voltage is negative.

When the power stage P operates half bridge, the obtained voltage provides a second range of currents G2 = [I21 - I22] smaller than the first range and in particular twice as small.

Thus, the power stage P is used to obtain the initial power PI via the calibration voltage UC but also the adjustment power PR via the functional voltage UF.

Thus, according to the desired current Iv that is to be obtained, the power stage P is used as full bridge (large range of currents G1) or half-bridge (small range of currents G2).

This makes it possible to obtain a magnetic field Be via the antenna device A regulated according to the desired type of vehicle. In fact, for example for family-type vehicles, a complete bridge will be used to provide a magnetic field emitted Be corresponding to a zone ZO delimiting the passenger compartment VH of this family vehicle, whereas for cut-off vehicles of which the cockpit is smaller, we will use a half bridge to provide a magnetic field emitted Be corresponding to this different and smaller cabin. Thus, thanks to this full-bridge or half-bridge operation, there is a field coverage adapted according to the type of vehicle without changing the RLC circuit in the antenna device A and therefore without having to adapt the resistance R of this circuit. The control device DC will be just programmed according to the vehicle type V to operate the power stage P adequately.

Moreover, it will be noted that for the same vehicle it may also be necessary to have a wide range of currents, for example in the case where there is a large variation of the battery voltage Ubat of the vehicle. Indeed, the adjustment power PR necessary to send the S_FONC functional signals is a function of this battery voltage and the impedance of the antenna device Zr. In order to compensate for the variations of the battery voltage Ubat, the adjustment duty cycle α2 is adjusted appropriately. For example, for a high battery voltage, the power stage P is operated half-bridge, while for a low battery voltage, it is operated full bridge. In order to obtain a continuous range between the two current ranges G1 and G2, in a non-limiting embodiment, the duty ratio α2 is in the range [1/6 - 1/2]. This is illustrated in Figure 15 showing a duty cycle-current diagram. On the abscissa is represented the cyclic ratio α, ordinate the current I. On the ordinate, we can see the respective limits I11, I12 and I21, I22 of the two ranges G1 and G2. When the cyclic ratio α2 varies in the interval [1/6 - 1/2], we can see that when one operates half-bridge 1 / 2H, one is on the curve CG2 of the second range of current G2. On the other hand, when operating with a full bridge H, it is on the curve CG1 of the first range G1. Finally, it can be seen that when switching from half-bridge to full-bridge operation, the range G2 to G1 is changed continuously, ie without jumping in current values I.

In another embodiment, if the duty cycle α2 varies in the interval [1/6 - α max '] with α max less than 1/2, it can be seen that there is a jump in the values of current when passing from the half bridge to the full bridge. In this case, some current values can not be taken into account to determine the power of the antenna device A. These are those in the interval I22 'and I22 hatched. In the latter mode, to ensure continuity, take the lower limit αmin of the interval less than 1/6.

• The current measuring device C.

In a first embodiment, the current measuring device is a peak amplitude detector. This is a simple way to measure the current. It makes it possible to measure the maximum amplitude of the current, which is sufficient because the disturbing harmonics have been suppressed by the symmetrical control and the duty cycle of 1/3. Thus, this measure will give the value of the fundamental of this current. It is conventionally composed of a diode and a capacitance.

It sends the value of the measured current Im to the calculator CALC tel.

Of course, other means for measuring the current can be used.

For example, the current measuring device C may be a digital sampling device or else a device which performs a rectification of current and then an average of the rectified current.

It will be noted that the antenna device A comprises one or more antennas. In the nonlimiting example described, it comprises a plurality of antennas as seen above. In this practical case, for each antenna of the antenna device A, the current is regulated in the antenna to obtain a corresponding nominal magnetic field B0 corresponding to the zone ZO for communication between the badge ID and the antenna. Thus, the ID badge comprises a plurality of fixed threshold value S0 associated with each antenna of the antenna device A.

Note that the example taken has been described with an indoor antenna. It will of course be possible to apply the method to an external antenna if necessary.

Moreover, it should be noted that the examples were taken with an antenna device A transmitting low frequency signals and an identification object ID transmitting radio frequency signals, but of course other examples can be taken with signal transmissions. at other frequencies.

• Elle permet de maîtriser la valeur du champ magnétique émis par le dispositif d'antennes A en régulant la puissance à l'émission ce qui permet d'obtenir un seuil fixe pour l'objet d'identification et est plus simple à gérer qu'un seuil variable pour ledit objet;
• En programmant l'objet d'identification ID avec un seuil fixe déterminé, cela permet d'éviter les perturbations radio et donc les champs magnétiques parasites;
• Par ailleurs, ce seuil est fixe pour tous les véhicules ce qui permet d'avoir un objet d'identification ID universel fonctionnant avec tous les véhicules, la zone de communication ZO étant adaptée seulement par la puissance d'émission Pe et donc par le courant I circulant dans le dispositif d'antennes A;
• La puissance est régulée au moyen d'une régulation de rapport cyclique qui est moins coûteuse qu'une régulation en tension avec rapport cyclique fixe;
• La régulation de rapport cyclique se fait en fonction du courant qui est plus efficace et précis qu'un réglage en fonction de la tension batterie car les variations de l'impédance Zr du dispositif d'antennes A sont compensées contrairement à une solution qui effectuerait une régulation de la tension d'alimentation Ubat;
• La commande en pont en H symétrique permet de ne pas émettre les harmoniques paires et par conséquent de réduire les problèmes de compatibilité électromagnétiques appelées CEM;
• La commande de calibration symétrique avec un rapport cyclique de 1/3 permet de réaliser une mesure précise du courant avec un moyen simple de mesure tel que le détecteur amplitude crête;
• La commande avec une tension carrée permet d'éviter aux transistors de l'étage de puissance de trop chauffer;
• Elle permet d'obtenir une large gamme de courants si nécessaire, pour un même véhicule ou pour des véhicules différents, sans avoir besoin d'adapter le circuit du dispositif d'antennes A;
• L'étape de calibration, qui permet de déterminer une puissance de réglage afin de déterminer la valeur du champ magnétique émis par le dispositif d'antenne(s), est dynamique car elle se fait pendant le fonctionnement du dispositif d'antenne(s);
• L'étape de calibration dynamique ne nécessite aucun appareil de mesure supplémentaire externe, contrairement à des étapes de calibration statiques qui se font en amont, c'est-à-dire pendant le montage d'un dispositif d'antenne(s) (c'est-à-dire lors de sa mise au point) dans un véhicule (donc avant même la mise en production et en vente du véhicule);
• Elle permet de maîtriser la valeur du champ magnétique émis par le dispositif d'antennes lors de son utilisation dans un véhicule en prenant en compte les variations de l'impédance du dispositif d'antenne(s) de manière à obtenir un seuil fixe pour l'objet d'identification; ces variations d'impédance apparaissant au cours de l'utilisation du dispositif d'antenne(s);
• Le système de détection comprenant le dispositif d'antenne(s) et l'objet d'identification permet d'effectuer une auto-calibration de dispositif d'antenne(s) sans appareil de mesure extérieur.

Thus, the invention has the following advantages:

• It makes it possible to control the value of the magnetic field emitted by the antenna device A by regulating the power at emission, which makes it possible to obtain a fixed threshold for the identification object and is simpler to manage than variable threshold for said object;
• By programming the identification object ID with a determined fixed threshold, this makes it possible to avoid radio disturbances and therefore parasitic magnetic fields;
• Moreover, this threshold is fixed for all the vehicles, which makes it possible to have a universal identification object ID that works with all the vehicles, the communication area ZO being adapted only by the transmission power Pe and therefore by the current I flowing in the antenna device A;
• The power is regulated by means of a duty cycle control which is less expensive than a voltage regulation with a fixed duty cycle;
• The duty cycle regulation is done according to the current which is more efficient and accurate than an adjustment according to the battery voltage because the variations of the impedance Zr of the antenna device A are compensated contrary to a solution which would perform a regulating the supply voltage Ubat;
• The symmetrical H-bridge control makes it possible not to emit the even harmonics and therefore to reduce electromagnetic compatibility problems called EMC;
• The symmetrical calibration command with a duty cycle of 1/3 allows precise measurement of the current with a simple measuring device such as the peak amplitude detector;
• The control with a square voltage prevents the transistors of the power stage from overheating;
• It makes it possible to obtain a wide range of currents if necessary, for the same vehicle or for different vehicles, without having to adapt the circuit of the antenna device A;
• The calibration step, which makes it possible to determine an adjustment power in order to determine the value of the magnetic field emitted by the antenna device (s), is dynamic because it is done during the operation of the antenna device (s) ;
• The dynamic calibration step does not require any additional external measuring device, unlike static calibration steps which are done upstream, ie during the mounting of an antenna device (s) (c). 'ie during its development) in a vehicle (thus even before the production and sale of the vehicle);
• It makes it possible to control the value of the magnetic field emitted by the antenna device when it is used in a vehicle by taking into account the variations of the impedance of the antenna device (s) so as to obtain a fixed threshold for the antenna. object of identification; these impedance variations occurring during the use of the antenna device (s);
• The detection system comprising the antenna device (s) and the identification object makes it possible to perform a self-calibration of antenna device (s) without an external measuring device.

Of course, the invention is not limited to the described application of the motor vehicle, but can be used in all applications involving a low frequency antenna and an identification object such as a home automation application for example.

Claims (19)

1. Method for detecting an identification object (ID) in a zone (ZO) around an antenna device (A) having one or more antennas, the method comprising, when the identification object (ID) is put into operation, a step of writing to the identification object a fixed threshold power value P0, corresponding to a nominal magnetic field B0, starting from which the identification object can receive signals from the antenna device and communicate with an associated control device, and comprising, in the operating mode of the identification object (ID) and of the antenna device, the steps of:

   - transmitting a calibration signal (S_CAL), corresponding to a theoretical initial current (Ith), towards the antenna device (A) to determine a setting power (PR),

   - measuring the current flowing in the antenna device during the transmission of the calibration signal (S_CAL),

   - comparing the measured current (Irm) with the theoretical initial current (Ith),

   - determining the setting power (PR) to be applied to the antenna device (A),

   - setting the setting power (PR) with an operating voltage (UF) having a given duty factor,

   - transmitting an operating signal (S_FONC) towards the antenna device (A) corresponding to the setting power (PR) in such a way that the antenna device transmits a specified magnetic field (Be),

   - measuring the magnetic field (Br) received by the identification object (ID) corresponding to the transmitted magnetic field and comparing it with the nominal magnetic field (B0),

   - determining whether or not the identification object (ID) is located in the zone around the antenna device (A), as a function of this comparison.
2. Detection method according to Claim 1, characterized in that the calibration signal (S_CAL) is not intelligible to the identification object (ID).
3. Detection method according to either of the preceding claims, characterized in that it comprises a supplementary step in which:

   - a current (Irm) flowing in the antenna device (A) is measured during the transmission of the calibration signal (S_CAL), and

   - the measured current (Irm) is compared with an initial current (Ith) in order to determine the setting power (PR).
4. Detection method according to any one of the preceding claims, characterized in that a power (PI, PR) is set with a voltage (UC, UF) having a given duty factor (α1, α2).
5. Detection method according to the preceding claim, characterized in that the voltage (UC) is a symmetrical signal.
6. Detection method according to one of Claims 4 and 5, characterized in that the duty factor (α1) is equal to 1/3.
7. Detection method according to any one of Claims 4 to 6, characterized in that the voltage (UC, UF) is generated by means of a power stage (P) with full-bridge or half-bridge control.
8. Detection method according to any of the preceding claims, characterized in that the calibration signal (S_CAL) is triggered as a function of a specific event.
9. Detection method according to the preceding claim, characterized in that the specific event is the accessing of a vehicle.
10. Detection method according to Claim 8, characterized in that the specific event is a variation in battery voltage.
11. Detection method according to any one of the preceding claims, characterized in that the fixed threshold value (S0) is a function of the nominal magnetic field (B0).
12. Detection method according to any one of the preceding claims, characterized in that the zone (ZO) around the antenna device is defined by the nominal magnetic field (B0).
13. Detection method according to any one of the preceding claims, characterized in that the zone around the antenna device (A) corresponds to a vehicle passenger compartment (VH).
14. System (SYS) for detecting an identification object (ID) in a zone (ZO) around an antenna device (A), comprising a control device (DC), an antenna device (A) and an identification object (ID), such that:

    - when the identification object (ID) is put into operation, a fixed threshold power value P0, corresponding to a nominal magnetic field B0, starting from which the identification object can receive signals from the antenna device and communicate with an associated control device, is written to the identification object (ID),

    - and such that the control device (DC) is, in the operating mode of the identification object (ID) and of the antenna device, capable of:

    - transmitting a calibration signal (S_CAL), corresponding to a theoretical initial current (Ith), towards the antenna device (A) to determine a setting power (PR),

    - measuring the current flowing in the antenna device during the transmission of the calibration signal (S_CAL),

    - comparing the measured current (Irm) with the theoretical initial current (Ith),

    - determining the setting power (PR) to be applied to the antenna device (A),

    - setting the setting power (PR) with an operating voltage (UF) having a given duty factor,

    - transmitting an operating signal (S_FONC) towards the antenna device (A) corresponding to the setting power (PR) in such a way that the antenna device transmits a specified magnetic field (Be),

    - determining whether or not the identification object (ID) is located in the zone around the antenna device (A), as a function of a comparison made between the magnetic field (Br) received by the identification object (ID) and a nominal magnetic field (B0),

    - the identification object (ID) is capable of measuring the received magnetic field (Br) corresponding to the transmitted magnetic field (Be) and comparing it with the nominal magnetic field.
15. Antenna device (A) capable of interacting with an identification object (ID), such that:

    - when the identification object (ID) is put into operation, a fixed threshold power value P0, corresponding to a nominal magnetic field B0, starting from which the identification object can receive signals from the antenna device and communicate with an associated control device, is written to the identification object (ID),

    - and such that, in the operating mode of the identification object (ID), it is capable of:

    - receiving a calibration signal (S_CAL), corresponding to a theoretical initial current (Ith) and to a specified initial power (PI),

    - measuring the current flowing in the antenna device during the transmission of the calibration signal (S_CAL),

    - comparing the measured current (Irm) with the theoretical initial current (Ith),

    - determining the setting power (PR) to be applied to the antenna device (A),

    - setting the setting power (PR) with an operating voltage (UF) having a given duty factor,

    - receiving an operating signal (S_FONC) corresponding to a specified setting power (PR) in such a way that a specified magnetic field (Be) is transmitted, and

    - transmitting the operating signal (S_FONC) to the identification object (ID), the latter receiving a magnetic field (Br) that is a function of the magnetic field (Be) transmitted by the antenna device (A).
16. Control device (DC) capable of interacting with an antenna device (A) and with an identification object (ID), such that, when the identification object (ID) is put into operation, a fixed threshold power value P0, corresponding to a nominal magnetic field B0, starting from which the identification object can receive signals from the antenna device and communicate with an associated control device, is written to the identification object (ID), and such that it comprises a signal transmitter (EM) for, in the operating mode of the identification object (ID),

    - transmitting a calibration signal (S_CAL), corresponding to a theoretical initial current (Ith), towards the antenna device (A) to determine a setting power (PR), and

    - measuring the current flowing in the antenna device during the transmission of the calibration signal (S_CAL),

    - comparing the measured current (Irm) with the theoretical initial current (Ith),

    - determining the setting power (PR) to be applied to the antenna device (A),

    - setting the setting power (PR) with an operating voltage (UF) having a given duty factor,

    - transmitting an operating signal (S_FONC) towards the antenna device (A) corresponding to the specified setting power (PR), in such a way that the antenna device (A) transmits a specified magnetic field (Be).
17. Control device (DC) according to the preceding claim, characterized in that it additionally comprises a signal receiver (RE) for receiving a response (REP) from the identification object (ID) as a function of a comparison made between a received magnetic field (Br) and a nominal magnetic field (B0).
18. Control device (DC) according to either of the preceding Claims 16 and 17, characterized in that the comparison is made by the identification object (ID).
19. Motor vehicle (V) comprising a passenger compartment (VH) in which a control device (DC) according to one of Claims 16 to 18 and an antenna device (A) according to Claim 15 are arranged, the two devices (DC, A) being able to interact with an identification object (ID).