Method for detecting the presence of an NFC transponder

Abstract

A method implemented in a near-field communication system (100) for detecting the presence of a radio frequency transponder (110) by a radio frequency reader (120) installed on a motor vehicle, said radio frequency transponder (110) being worn by a user located outside the vehicle, and said radio frequency reader (120) being intended to communicate with the radio frequency transponder (110) for controlling access to the motor vehicle, the method comprising the following steps implemented by the radio frequency reader: - transmission of a radio frequency interrogation signal; then - transmission of a radio frequency feed signal, intended to be received and then modulated by the radio frequency transponder; - detection of the presence of the radio frequency transponder (110), by measuring the amplitude of the radio frequency feed signal and comparing it to at least a predetermined threshold. Figure for the abbreviation: Figure 1

Description

Title of the invention: Method for detecting the presence of an NFC transponder. Technical field

[0001] The invention relates to the automotive field and more particularly to the field of near field communication (or NFC for the English "Near Field Communication") systems allowing an exchange of data between a transponder worn by a user and an on-board vehicle access management system. State of the art

[0002] Near field communication systems for use in the automotive field are known in the prior art, for the purpose of authorizing or denying access to the vehicle based on the identity of the approaching user.

[0003] Such a system includes, in particular, a radio frequency reader, installed in the motor vehicle, and a radio frequency transponder, worn by a user outside the vehicle and integrated into a badge or smartphone. The radio frequency reader is configured to query the radio frequency transponder to obtain at least one authentication code stored in the latter. This authentication code is transmitted to a vehicle access management system, which determines whether or not the radio frequency transponder is authorized to access the vehicle.Depending on the vehicle access permissions stored in the access management system, at least one vehicle access command is generated or not, preferably a command to lock or unlock at least one vehicle opening and / or a command to open or close at least one vehicle opening and / or a command to open or close at least one vehicle window.

[0004] In a manner known per se, the method implemented at the radio frequency reader includes an initial step of detecting the presence of a radio frequency transponder by emitting short pulses dedicated to said detection and measuring the possible influence of a radio frequency transponder located in the vicinity of the radio frequency reader. This presence detection is known as LPCD detection, for the English "Low Power Card Detection".

[0005] Once the presence of a radio frequency transponder has been detected near the radio frequency reader, the radio frequency reader emits a modulated radio frequency interrogation signal, forming a request to read data from the radio frequency transponder. Different modulations can be implemented: amplitude modulation and / or phase modulation and / or frequency modulation. In particular, several are known. amplitude modulation solutions, characterized by a waveform (shape of the unmodulated signal, for example a continuous signal, or a square wave, or any other shape) and a ratio between the maximum peak-to-peak amplitude (encoding a bit with a value equal to one) and the minimum peak-to-peak amplitude (encoding a bit with a value of zero).

[0006] After this, the radio frequency reader emits a power radio frequency signal, intended to be received by the radio frequency transponder. Indeed, the radio frequency reader is configured to be able to communicate with any type of radio frequency transponder, whether active or passive. An active radio frequency transponder has a power source for generating a reply signal, sent in response to the reception of the interrogation radio frequency signal. A passive radio frequency transponder, on the other hand, does not have such a power source. To transmit data, it can only modulate a signal that it receives, in this case, the aforementioned power radio frequency signal.

[0007] In any event, from the moment the power radio frequency signal is transmitted, the radio frequency reader normally receives a return signal modulated by the radio frequency transponder. This may be a signal generated by the passive transponder, or the power radio frequency signal influenced, in other words modulated, by the radio frequency transponder.

[0008] The feed radio frequency signal is a continuous signal with a constant amplitude in the absence of environmental influence, particularly from the radio frequency transponder. The feed radio frequency signal is commonly referred to as the "FWI" frame, short for "Frame Waiting Time Integer." The term "FWI" also designates an integer whose value defines the FWT (Frame Waiting Time) of the "FWI" frame. In particular, FWI takes a value between 0 and 14, preferably 4. The FWT is defined by: FWT = (256 * 16 / fc) * 2FWI, where fc is the carrier frequency of the radio frequency signal. The FWT is generally between 300 ps and 5 s.

[0009] The near-field communication between the radio frequency reader and the radio frequency transponder advantageously consists of an alternation of radio frequency interrogation signals and radio frequency power supply signals.

[0010] It may happen that the radio frequency transponder leaves the immediate vicinity of the radio frequency reader while the communication process is in progress, that is, after the transmission of the feed radio frequency signal. In this case, the radio frequency reader must wait for the entire predetermined duration of the feed radio frequency signal (FWI frame), and then implement a timeout error confirmation process if the radio frequency reader has not received any modulated return signal during the entire duration of this TWI frame.

[0011] This process for confirming a time-out error consists of: - send a new, specific interrogation signal, - wait for a response for a predetermined time, emitting a power signal during this waiting period, and - repeat these operations several times, until a valid response is finally received from the radio frequency transponder or until a predetermined number of unsuccessful attempts has been reached.

[0012] In the second case, where the predetermined number of attempts has been reached without receiving any valid response, a timeout error is generated on the radio frequency reader side, which terminates the communication initiated between the radio frequency reader and the transponder. This timeout confirmation process can last several seconds, for example, 5 x 4.95 s ≈ 25 s for a number of attempts equal to 5 and a FWT duration as defined above equal to 4.95 s. Furthermore, this process involves the transmission of a radio frequency signal during this entire time, which consumes energy.

[0013] It can also happen that the radio frequency transponder leaves the immediate vicinity of the radio frequency reader before the communication process has even started. In this case, the process is roughly the same, with the implementation of a time-out error confirmation process that takes time and consumes energy.

[0014] An objective of the present invention is to provide a faster and more energy-efficient solution for managing a situation in which the radio frequency reader does not receive any modulated return signal during the entire duration of the transmission of the power radio frequency signal (FWI frame). Description of the invention

[0015] This objective is achieved with a method implemented in a near-field communication system, for detecting the presence of a radio frequency transponder by a radio frequency reader mounted on a motor vehicle, said radio frequency transponder being worn in use by a user located outside the vehicle, and said radio frequency reader being intended to communicate with the radio frequency transponder for controlling access to the motor vehicle, the method comprising the following steps implemented by the radio frequency reader: - transmission of a modulated radio frequency interrogation signal, forming a request to read data from the radio frequency transponder; then - emission of a radio frequency power signal, intended to be received and then modulated by the radio frequency transponder for the transmission of the data requested by the radio frequency interrogation signal.

[0016] According to the invention, the method further comprises a step of detecting the presence of the radio frequency transponder, by measuring an amplitude of the radio frequency supply signal and comparing it to at least a predetermined threshold.

[0017] Advantageously but not limitingly, this presence detection step is started at the same time as the step of emitting a radio frequency power signal, so as to be able to detect as early as possible that the radio frequency transponder has left the near field of the radio frequency reader.

[0018] The method according to the invention offers a solution for confirming the presence or absence of the radio frequency transponder, in a faster and more precise manner than using the time-over error confirmation steps described above.

[0019] It may happen that the radio frequency transponder does not respond to the power supply radio frequency signal, but is nevertheless present in the environment of the radio frequency reader. This is the case, for example, in the event of electromagnetic interference in the near field of the radio frequency reader, or if the radio frequency transponder is poorly positioned, or if the radio frequency transponder is of the active type and lacks the energy to emit a response signal. The invention exploits the fact that, in such a situation where the radio frequency transponder does not respond to the power supply radio frequency signal while being present in the environment of the radio frequency reader, it will influence the amplitude of said power supply radio frequency signal.Measuring this amplitude and comparing it to at least a predetermined threshold provides information on whether or not the radio frequency transponder is present near the radio frequency reader. This allows us to confirm or refute a suspicion that the radio frequency transponder has left the immediate vicinity of the radio frequency reader.

[0020] Throughout the text, the notion of immediate environment refers to the range of a radio frequency reader of a near field communication system, of the "NFC" type, i.e. a radius of a few centimeters, for example less than ten centimeters around the radio frequency reader.

[0021] One of the advantages of the method according to the invention is that, for presence detection, it uses a radio frequency signal necessarily emitted following the transmission of the interrogation radio frequency signal: the feed radio frequency signal (or FWI frame, as described in the introduction). Therefore, implementing the method does not require any substantial modification of existing equipment. Furthermore, since the radio frequency signal used for presence detection is an unmodulated signal (in the absence of a radio frequency transponder near the radio frequency reader), detecting a variation in amplitude is particularly easy.

[0022] The method according to the invention thus offers a solution for confirming the presence or absence of the radio frequency transponder more quickly and accurately than using the time-over error confirmation steps described above. Furthermore, the method according to the invention is particularly energy-efficient, since it does not require any additional signal transmission. Subject to applicable standards, it would then be possible to dispense with the procedure described in the introduction, which is both lengthy and energy-intensive because it requires several signal transmission cycles by the radio frequency reader, and thus drastically reduce the power consumption of the near-field communication system.

[0023] Preferably, the presence detection includes a measurement of the amplitude of the power supply radio frequency signal, and a comparison relative to a value of the amplitude of the power supply radio frequency signal in the absence of a radio frequency transponder, called the no-load value.

[0024] Advantageously, the presence of the radio frequency transponder is detected when a difference between the measured value of the amplitude of the power supply radio frequency signal and said no-load value is greater than a predetermined deviation threshold.

[0025] Alternatively, the presence of the radio frequency transponder can be detected when a ratio between the measured value of the amplitude of the power radio frequency signal and said no-load value is less than a predetermined ratio threshold.

[0026] According to another variant, the presence of the radio frequency transponder can be detected when a ratio between a difference between the measured value of the amplitude of the power radio frequency signal and said no-load value, on the one hand, and said no-load value, on the other hand, is less than a predetermined ratio threshold.

[0027] Preferably, the amplitude of the radio frequency power supply signal remains greater than or equal to half the no-load value, even in the presence of the radio frequency transponder.

[0028] The radio frequency power supply signal advantageously has a constant amplitude in the absence of a radio frequency transponder.

[0029] The radio frequency power supply signal can have a duration of between 250 microseconds and 6 seconds.

[0030] The method according to the invention is advantageously implemented with a passive type radio frequency transponder.

[0031] Preferably, the signal emission by the radio frequency reader is suspended when the absence of the radio frequency transponder is detected at the end of the radio frequency transponder presence detection step.

[0032] The method may further include receiving a return radio frequency signal, corresponding to the power supply radio frequency signal modulated by the transponder radio frequency, and containing radio frequency transponder authentication information intended to be used to authorize or deny access to the motor vehicle.

[0033] The invention also relates to a use of the method according to the invention, which is implemented after a first switching from a locked or unlocked state of at least one opening of the motor vehicle, respectively a switching from an open or closed state of at least one opening of the motor vehicle, respectively a switching from an open or closed state of at least one window of the motor vehicle, in response to an authentication of the radio frequency transponder, in which a new switching of this state is only authorized if the absence of said radio frequency transponder has been detected at the end of the radio frequency transponder presence detection step.

[0034] The invention also relates to a use of the method according to the invention, which is implemented to detect a back-and-forth movement of the radio frequency transponder relative to the radio frequency reader, said movement being associated with a command to switch from a locked or unlocked state of at least one opening of the motor vehicle, respectively to switch from an open or closed state of at least one opening of the motor vehicle, respectively to switch from an open or closed state of at least one window of the motor vehicle.

[0035] The invention finally covers a radio frequency reader intended to be mounted on a motor vehicle for communication with a radio frequency transponder worn in use by a user located outside the vehicle, said radio frequency reader being intended to communicate with the radio frequency transponder for the control of access to the motor vehicle, and being configured to implement the steps of the method according to the invention. Description of the figures

[0036] Other features and advantages of the invention will become apparent from the following description. This description is purely illustrative and should be read in conjunction with the accompanying drawings, in which:

[0037] [Fig-1] Fig. 1 schematically illustrates a communication system in near field in which a presence detection method according to the invention is implemented;

[0038] [Fig.2] Figure [Fig.2] schematically illustrates the steps of a process according to the invention; and

[0039] [Fig.3] Fig.3 schematically illustrates the amplitude as a function of time of a radio frequency signal emitted by a radio frequency reader implementing a method according to the invention.

[0040] Detailed description of at least one embodiment

[0041] First, with reference to [Fig.1], a near field communication system 100 is described in which a presence detection method according to the invention is implemented.

[0042] The system 100 consists of a radio frequency transponder 110 and a radio frequency reader 120.

[0043] The term "near field communication" refers to a short-range, high-frequency wireless communication technology (preferably between 5 MHz and 20 MHz, for example approximately 14 MHz) that allows the exchange of information between devices (here, the radio frequency transponder 110 and the radio frequency reader 120) up to a distance of approximately ten centimeters. Near field communication preferably conforms to standards known as NFC (Near Field Communication).

[0044] In use, the radio frequency reader 120 is mounted on a motor vehicle (in [Fig.1], the interface 10 between the vehicle and its external environment is schematically represented).

[0045] The radio frequency reader 120 includes in particular at least one antenna 121, an adaptation circuit 122, a signal pre-processing chip 123, and a microcontroller 124 connected together in that order.

[0046] The radio frequency antenna 120, referred to as the NFC antenna 121, is positioned in use near an external surface of the vehicle, for example at a door handle or a vertical structural pillar located between a front door and a rear door. The NFC antenna 121 is configured to transmit and receive a radio frequency signal, in particular a near-field communication signal.

[0047] The matching circuit 122 is configured to perform impedance matching between the impedance of the NFC antenna 121 and the impedance of the circuits on the signal pre-processing chip 123. The matching circuit 122 advantageously comprises at least two metallic tracks, which extend on a printed circuit board each between a respective end of the NFC antenna 121 and the signal pre-processing area 123.

[0048] The signal preprocessing chip 123 includes a clock at the frequency of the radio frequency signal to be transmitted, for generating an electrical signal intended to be sent to the input of the NFC antenna 121. The signal preprocessing chip 123 also includes two mixers, each configured to mix a signal received by the NFC antenna 121 with the in-phase clock signal, respectively in quadrature phase, so as to obtain an I signal and a Q signal. The signal preprocessing chip 123 also includes at least one analog-to-digital converter, for performing time sampling of a signal.

[0049] The signal pre-processing chip 123 is configured to provide said time-sampled I and Q signals to the microcontroller 124. The microcontroller 124 is configured to perform the analysis of said signals so as to extract amplitude and / or phase and / or frequency data, and to implement a process as described below.

[0050] The amplitude and / or phase and / or frequency data can encode authentication information that can be extracted directly within the microcontroller 124, or within a remote computer (not shown).

[0051] In use, the radio frequency transponder 110 is worn by a user, in particular a user located outside the vehicle and wishing to access said vehicle. The radio frequency transponder 110 may take the form of a dedicated badge, or "keyfob," or be an integral part of a smart phone, or "smartphone." The radio frequency transponder 110 comprises, at a minimum, an NFC antenna 111 and a memory 112.

[0052] The NFC antenna 111 is configured to perform near field communication with the antenna 121 of the radio frequency reader 120. The memory 112 advantageously stores at least one authentication data.

[0053] The system 100, comprising the radio frequency transponder 110 and the radio frequency reader 120, belongs to a vehicle access system configured to allow vehicle access only to users equipped with an authorized radio frequency transponder 110. More specifically, the near-field communication system 100 is configured, in operation, to establish near-field communication, via the radio frequency reader 120, between the radio frequency transponder 110 and a computer forming a vehicle access management system, configured to compare an authentication code stored in the radio frequency transponder 110 and communicated via the radio frequency reader 120, with an authentication code associated with at least one authorized transponder and stored in the memory of a vehicle access system.When the authentication codes match, the vehicle access management system generates at least one vehicle access command, preferably a command to lock or unlock at least one vehicle opening and / or a command to open or close at least one vehicle opening and / or a command to open or close at least one vehicle window. Throughout this text, "opening" refers to a front or rear door or tailgate, or a front or rear trunk lid opening.

[0054] Advantageously, but not limitingly, the radio frequency transponder 110 is a passive type. This means that it does not have a power source to generate its own radio frequency signal. It transmits information by modifying a radio frequency signal generated by the radio frequency reader 120. In particular, the radio frequency transponder 110 is then configured to modulate a radio frequency signal provided by the radio frequency reader 120, so as to encode data such as authentication data. The modulation is advantageously amplitude modulation.

[0055] The steps of a process according to the invention are now described with reference to Figures 2 and 3.

[0056] Figure 2 illustrates more particularly the steps of a process according to the invention, implemented in a near field communication system 100.

[0057] Fig. 3 illustrates more particularly, and schematically, the evolution over time of the amplitude of a radio frequency signal emitted by the radio frequency reader 120.

[0058] In a preliminary step, and in a manner known per se, the radio frequency reader 120 performs a so-called low power presence detection, or LPCD for "Low Power Card Detection". This presence detection consists of emitting very short, time-spaced radio frequency pulses and detecting a change in at least one parameter among the amplitude, phase, and frequency indicating the presence of a radio frequency transponder 110 in the near field (in practice, a radius of ten centimeters or less) of the radio frequency reader 120. In [Fig. 3], the LPCD pulses correspond to the signal portion 31, and the entry of the radio frequency transponder 110 into the near field of the radio frequency reader 120 is marked by a change in the amplitude of the pulses (transition from an amplitude A1 to an amplitude A2).

[0059] In response to this presence detection, the radio frequency reader 120 switches from standby mode to active mode, and starts a near field communication with the radio frequency transponder 110.

[0060] In a first step 21 of this communication, the radio frequency reader 120 emits a radio frequency interrogation signal 32 constituting a data request from the radio frequency transponder 110. As detailed in the introduction, different modulations can be implemented: amplitude and / or phase and / or frequency modulation. Figure 3 illustrates in particular the example of amplitude modulation. As explained in the introduction, and in a manner known per se, amplitude modulation is characterized by a waveform and by a ratio between the maximum peak-to-peak amplitude (encoding a bit with a value equal to one) and the minimum peak-to-peak amplitude (encoding a bit with a value of zero).

[0061] In a second step 22 of this communication, the radio frequency reader 120 emits an unmodulated radio frequency power supply signal 33. The radio frequency power supply signal 33 is intended to be received and then modulated (in amplitude and / or in phase and / or in frequency) by the radio frequency transponder 110, for the transmission of the data requested by the radio frequency interrogation signal, preferably authentication data.

[0062] The feed radio frequency signal 33 is suitable for a passive radio frequency transponder. Provided that the radio frequency transponder 110 is passive or active, the radio frequency reader 120 transmits such a feed radio frequency signal after a sequence of transmitting an interrogation radio frequency signal. As detailed in the introduction, the feed radio frequency signal 33 is also known as the "FWI frame".

[0063] According to the invention, step 22 is followed by a step 23 of detecting the presence of the radio frequency transponder 110, by measuring an amplitude of the power supply radio frequency signal 33 and comparing it to at least a predetermined threshold.

[0064] In other words, said presence detection step 23 includes a step 230 of measuring an amplitude of the radio frequency supply signal, a step 231 of comparing between a quantity that is a function of said measured amplitude and a predetermined threshold, and finally a step 232 of determining the presence or absence of the radio frequency transponder 110 in the near field of the radio frequency reader 120.

[0065] These steps are implemented by the microcontroller 124 of the radio frequency reader 120.

[0066] Advantageously, the presence detection step 23 starts simultaneously with the power supply radio frequency signal transmission step 22, so as to detect the absence of the radio frequency transponder as early as possible. This rapid presence detection is particularly advantageous, especially when using the method to detect back-and-forth movement of the radio frequency transponder relative to the radio frequency reader (tapping). Such movement can control a vehicle function, in particular a vehicle access function such as those described above (locking, unlocking, opening a door, closing a door, opening a window, closing a window).

[0067] The power supply radio frequency signal 33 has a duration of between 250 microseconds and 6 seconds. It can be assumed that the predetermined threshold is on the order of half this duration, for example between 100 microseconds and 3 seconds.

[0068] The method according to the invention thus proposes to perform presence detection as early as possible. When communication standards allow it, this can avoid unnecessary signal emissions by the radio frequency reader 120 and the associated power consumption. In particular, it is proposed to cleverly exploit the fact that the power supply radio frequency signal 33 is an unmodulated signal, the amplitude of which is therefore mainly affected by the presence or absence of the radio frequency transponder 110 in the near field of the radio frequency reader. 120. In other words, the power supply radio frequency signal 33 has a constant amplitude in the absence of a radio frequency transponder. Its amplitude can be affected by amplitude modulation controlled by the radio frequency transponder 110, or by the mere presence of the radio frequency transponder in the absence of modulation by the latter.

[0069] Advantageously, presence detection includes a measurement of the amplitude of the power supply radio frequency signal, and a comparison relative to a value of the amplitude of the power supply radio frequency signal in the absence of a radio frequency transponder, called the no-load value.

[0070] The presence of the radio frequency transponder 110 in the near field of the radio frequency reader 120 modifies the amplitude of the measured power supply radio frequency signal compared to its no-load value, in the absence of the radio frequency transponder 110 in the near field of the radio frequency reader 120. The presence of the radio frequency transponder 110 in the near field of the radio frequency reader 120 can be detected based on an absolute difference, or gap, between the measured amplitude and the no-load amplitude. Alternatively, the presence of the radio frequency transponder 110 in the near field of the radio frequency reader 120 can be detected based on a ratio, or relationship, between the measured amplitude and the no-load amplitude. A ratio between said gap and the no-load amplitude can also be used.One or the other of these criteria will be advantageously chosen by a person skilled in the art, depending on the expected difference, the no-load range, and in such a way as to minimize a false detection rate.

[0071] In any event, the influence of the presence of the radio frequency transponder 110 on the amplitude of the power supply radio frequency signal remains relatively small. In particular, the amplitude of the power supply radio frequency signal 33 remains greater than or equal to half, and even 80%, of its no-load value, even in the presence of the radio frequency transponder 110.

[0072] Advantageously, but optionally, when the absence of the radio frequency transponder 110 in the near field of the radio frequency reader 120 is detected in step 23, the signal transmission by said radio frequency reader is suspended, and the latter may even switch back to standby mode. This minimizes the power consumption of the radio frequency reader 120, which is a definite advantage for a component installed in a motor vehicle.

[0073] Conversely, when the presence of the radio frequency transponder 110 is detected in the near field of the radio frequency reader 120 in step 23, the method advantageously then comprises receiving a return radio frequency signal, corresponding to the feed radio frequency signal 33 modulated by the radio frequency transponder 110 or to a radio frequency signal generated by the active transponder. The return radio frequency signal contains a transponder authentication code. Radio frequency 110 is intended to be extracted and analyzed by the vehicle access management system described above to authorize or deny access to the motor vehicle. Specifically, at least one action is authorized or denied, including locking or unlocking at least one vehicle door, opening or closing at least one vehicle door, and opening or closing at least one vehicle window.

[0074] The invention finds a particularly advantageous application in quickly confirming, with minimal signal, that the radio frequency transponder 110 has left the near field of the radio frequency reader 120, after an initial change in the locked or unlocked state of at least one opening of the motor vehicle (from locked to unlocked, or vice versa), or the open or closed state of at least one opening of the motor vehicle (from open to closed, or vice versa), or the open or closed state of at least one window of the motor vehicle (from open to closed, or vice versa). The aim is to allow a further change in this state only if the absence of the radio frequency transponder 110 has been detected in the meantime.In other words, a new switchover to this state is only permitted if it is confirmed that the radio frequency transponder 110 has left the near field of the radio frequency reader 120 since the previous switchover. The objective is to avoid unwanted state switches, with several successive state changes while the radio frequency transponder 110 has not left the near field of the radio frequency reader 120.

[0075] The invention thus proposes a solution for detecting the presence of an external NFC device (the radio frequency transponder 110), without it being necessary to wait for the end of the communication.

[0076] One of the objectives is to only allow a new communication with the same external NFC device (the radio frequency transponder 110), and thus a new switching, in particular locking or unlocking of at least one opening of the motor vehicle, if said external NFC device has meanwhile left the near field of the NFC reader (radio frequency reader 120).

Claims

Demands

1. A method implemented in a near field communication system (100), for detecting the presence of a radio frequency transponder (110) by a radio frequency reader (120) mounted on a motor vehicle, said radio frequency transponder (110) being carried in use by a user located outside the vehicle, and said radio frequency reader (120) being intended to communicate with the radio frequency transponder (110) for controlling access to the motor vehicle, the method comprising the following steps implemented by the radio frequency reader: - emission (21) of a modulated radio frequency interrogation signal (32) forming a request to read data from the radio frequency transponder (110); then - emission (22) of a radio frequency power signal (33), intended to be received and then modulated by the radio frequency transponder (110) for the transmission of the data requested by the radio frequency interrogation signal;characterized in that it further comprises a step (23) detection of the presence of the radio frequency transponder (110), by measuring an amplitude of the radio frequency supply signal and comparing it to at least a predetermined threshold.;

2. A method according to claim 1, characterized in that the presence detection (23) includes a measurement of the amplitude of the power radio frequency signal (33), and a comparison with respect to a value of the amplitude of the power radio frequency signal in the absence of a radio frequency transponder, called the no-load value.

3. Method according to claim 2, characterized in that the presence of the radio frequency transponder (110) is detected when a difference between the measured value of the amplitude of the power supply radio frequency signal (33) and said no-load value is greater than a predetermined difference threshold.

4. Method according to claim 2, characterized in that the presence of the radio frequency transponder (110) is detected when a ratio between the measured value of the amplitude of the power radio frequency signal and said no-load value is less than a predetermined ratio threshold.

5. A method according to claim 2, characterized in that the presence of the radio frequency transponder (110) is detected when a ratio between a difference between the measured value of the amplitude of the power radio frequency signal (33) and said no-load value, on the one hand, and said no-load value, on the other hand, is less than a predetermined ratio threshold.

6. A method according to any one of claims 1 to 5, characterized in that the amplitude of the radio frequency power supply signal (33) remains greater than or equal to half the no-load value, even in the presence of the radio frequency transponder (110).

7. A method according to any one of claims 1 to 6, characterized in that the radio frequency power supply signal (33) has a constant amplitude in the absence of a radio frequency transponder (110).

8. Method according to claim 7, characterized in that the feed radio frequency signal (33) has a duration between 250 microseconds and 6 seconds.

9. A method according to any one of claims 1 to 8, characterized in that it is implemented with a passive radio frequency transponder (110).

10. A method according to any one of claims 1 to 9, characterized in that the signal emission by the radio frequency reader (120) is suspended when the absence of the radio frequency transponder (110) is detected at the end of step (23) detection of the presence of the radio frequency transponder (110).

11. A method according to any one of claims 1 to 10, characterized in that it further comprises the reception of a return radio frequency signal, corresponding to the supply radio frequency signal (33) modulated by the radio frequency transponder (110), and containing authentication information for the radio frequency transponder (110) intended to be used to authorize or deny access to the motor vehicle.

12. Use of the method according to any one of claims 1 to 11, implemented after a first switching from a locked or unlocked state of at least one opening of the motor vehicle, or respectively a switching from an open or closed state of at least one opening of the motor vehicle, or respectively a switching from an open or closed state of at least one window of the vehicle automobile, in response to an authentication of the radio frequency transponder (110), in which a new switch of this state is only permitted if the absence of said radio frequency transponder (110) has been detected at the end of step (23) detection of presence of the radio frequency transponder (110).

13. Use of the method according to any one of claims 1 to 11, implemented to detect a back-and-forth movement of the radio frequency transponder (110) relative to the radio frequency reader (120), said movement being associated with a command to switch at least one opening of the motor vehicle from a locked or unlocked state, or to switch at least one opening of the motor vehicle from an open or closed state, or to switch at least one window of the motor vehicle from an open or closed state.

14. Radio frequency reader (120) intended to be mounted on a motor vehicle for communication with a radio frequency transponder (110) worn in use by a user located outside the vehicle, said radio frequency reader (120) being intended to communicate with the radio frequency transponder (110) for controlling access to the motor vehicle, and being configured to implement the steps of the method according to any one of claims 1 to 11.

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