Method for detecting presence of NFC transponder

Abstract

A method for detecting the presence of an NFC transponder. A method implemented in a near field communication system (100) for detecting the presence of a radio frequency transponder (110) by means of a radio frequency reader (120) 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) in order to control access to the motor vehicle, the method comprises the following steps implemented by a radio frequency reader:-transmitting a radio frequency interrogation signal; then-transmitting a radio frequency supply signal intended to be received by the radio frequency transponder and then modulated; -detecting the presence of a radio frequency transponder (110) by measuring the amplitude of the radio frequency supply signal and comparing it with at least one predetermined threshold.

Description

Technical Field

[0001] This invention relates to the field of automobiles, and more particularly to the field of near field communication (NFC) systems, which allow data exchange between a user-carried transponder and an on-board system for managing access to the vehicle. Background Technology

[0002] Near-field communication systems designed for use in the automotive field are known in the prior art, with the aim of authorizing or disauthorizing access to a vehicle based on the identity of the user in the vicinity.

[0003] Such systems specifically include radio frequency (RF) readers on motor vehicles in use, and RF transponders carried by users located outside the vehicle and integrated into badges or smartphones. The RF reader is configured to interrogate the RF transponder to obtain at least one authentication code stored therein. This authentication code is transmitted to a vehicle access management system, which determines whether the RF transponder is authorized to enter the vehicle. Depending on the vehicle access authorization stored in the access management system, at least one vehicle entry command may or may not be generated, preferably a command for locking or unlocking at least one opening element of the vehicle and / or a command for opening or closing at least one opening element of the vehicle and / or a command for opening or closing at least one window of the vehicle.

[0004] A method implemented at an RF reader in a manner known per se includes the initial step of detecting the presence of an RF transponder by transmitting a short-duration pulse dedicated to the detection and measuring the potential impact of an RF transponder located near the RF reader. This presence detection is known as LPCD detection, which stands for "Low Power Card Detection".

[0005] Once the presence of an RF transponder is detected near the RF reader, the RF reader transmits an RF interrogation signal, which is modulated to form a request to read data from the RF transponder. Various modulations can be implemented: amplitude and / or phase and / or frequency modulation. In particular, various amplitude modulation solutions are known, characterized by the waveform (the shape of the unmodulated signal, such as a continuous signal, or a square wave, or any other shape) and the ratio between the maximum peak-to-peak amplitude (encoding bits equal to a unit value) and the minimum peak-to-peak amplitude (encoding zero-value bits).

[0006] Subsequently, the RF reader transmits an RF power signal intended to be received by the RF transponder. Specifically, the RF reader is configured to communicate with any type of RF transponder, whether active or passive. Active RF transponders include an energy source for generating a response signal sent in response to a received RF interrogation signal. Passive RF transponders, on the other hand, do not include such an energy source. To transmit data, it can modulate only the signal it receives, in this case, the RF supply signal.

[0007] In any case, starting from the transmission of the RF supply signal, the RF reader typically receives a return signal modulated by the RF transponder. Therefore, it can be a signal generated by a passive transponder or, in other words, a modulated RF supply signal influenced by the RF transponder.

[0008] The radio frequency (RF) supply signal is a continuous signal with a constant amplitude in the absence of environmental influences, particularly from the RF transponder. The RF supply signal is commonly referred to as a "FWI" frame, which stands for "Frame Wait Time Integer." The term "FWI" also refers to the integer value that defines the FWT (Frame Wait Time) duration of the "FWI" frame. Specifically, FWI takes a value between 0 and 14, preferably 4. The FWT duration is calculated as: FWT = (256 * 16 / f c )*2 FWI Define f c It is the carrier frequency of the radio frequency signal. The duration of FWT is generally between 300μs and 5s.

[0009] Near-field communication between radio frequency readers and radio frequency transponders advantageously consists of alternation between radio frequency interrogation signals and radio frequency supply signals.

[0010] While communication is in progress, i.e. after the radio frequency power signal has been transmitted, it is possible that the radio frequency transponder leaves the environment close to the radio frequency reader. In this case, the radio frequency reader must wait for the entire predetermined duration of the radio frequency supply signal (FWI frame), and then, if the radio frequency reader does not receive any modulated return signal during the entire duration of the TWI frame, it will acknowledge the timeout error.

[0011] The process for confirming a timeout error includes:

[0012] - Send a new specific query signal

[0013] - The waiting period for a response is predetermined, with the transmission of a supply signal for that waiting period, and

[0014] - Repeat these steps several times until a valid response is finally received from the radio frequency transponder or until a predetermined number of unsuccessful attempts have been reached.

[0015] In the second scenario, where a predetermined number of attempts have been made without receiving any valid response, a timeout error is generated on the RF reader side, terminating the communication initiated between the RF reader and the transponder. This confirmation of the timeout error may last for several seconds; for example, for a number of attempts equal to 5, 5 × 4.95 s ≈ 25 s, and the FWT duration as defined above is equal to 4.95 s. Furthermore, throughout this entire time, the process involves the energy-consuming transmission of an RF signal.

[0016] It is also possible that the RF transponder leaves the environment close to the RF reader even before the communication process begins. In this case, the process is largely the same, with an implementation of a process for confirming timeout errors, which takes time and consumes energy.

[0017] The purpose of this invention is to provide a faster and more energy-efficient technical solution for managing situations in which the radio frequency reader does not receive a modulated return signal during the entire duration of continuous radio frequency supply signal (FWI frame) transmission. Summary of the Invention

[0018] This objective is achieved using a method implemented in a near-field communication system for detecting the presence of an RF transponder by means of an RF reader on a motor vehicle, the RF transponder being carried in use by a user located outside the vehicle, and the RF reader being designed to communicate with the RF transponder to control access to the motor vehicle. The method includes the following steps implemented by the RF reader:

[0019] - Transmit an radio frequency interrogation signal, which is modulated and forms a request to read data from the radio frequency transponder; then

[0020] - Transmit an RF supply signal, which is intended to be received by an RF transponder and then modulated for the transmission of data requested by an RF interrogation signal.

[0021] According to the present invention, the method further includes the step of detecting the presence of a radio frequency transponder by measuring the amplitude of a radio frequency supply signal and comparing it with at least one predetermined threshold.

[0022] Advantageously, but not limitingly, the presence detection step begins at the same time as the step of transmitting the radio frequency supply signal, so as to be able to detect as quickly as possible that the radio frequency transponder has left the near field of the radio frequency reader.

[0023] According to the method of the present invention, a technical solution is proposed for confirming the presence or absence of an RF transponder faster and more accurately than using the steps described above for confirming timeout errors.

[0024] It is possible that an RF transponder may not respond to an RF supply signal, yet it still exists in the environment of an RF reader. This can be due to electromagnetic interference in the near field of the RF reader, improper positioning of the RF transponder, or, in other words, if the RF transponder is active and lacks the energy to transmit a response signal. This invention utilizes the fact that in such a case, where the RF transponder does not respond to the RF supply signal while in the environment of the RF reader, it will affect the amplitude of the RF supply signal. Measuring this amplitude and comparing it to at least one predetermined threshold makes it possible to obtain information about the presence of an RF transponder near the RF reader. Therefore, this can confirm or dispel suspicion that the RF transponder has left the immediate environment of the RF reader.

[0025] Throughout the text, the concept of direct environment refers to the range of the radio frequency reader in a near field communication (NFC) system, that is, the radius of the radio frequency reader around the radio frequency reader, for example, less than ten centimeters.

[0026] One of the key features of the method according to the invention is that, for presence detection, a radio frequency (RF) signal necessary to be transmitted after the RF interrogation signal is transmitted is used: an RF supply signal (or an FWI frame, as described in the introduction). Therefore, implementing this method does not require any substantial modification to existing methods. Furthermore, since the RF signal used for presence detection is an unmodulated signal (in the absence of an RF transponder near the RF reader), the detection of amplitude changes is particularly easy to achieve.

[0027] Therefore, the method according to the invention provides a faster and more accurate way to confirm the presence or absence of an RF transponder than using the confirmation timeout error steps described above. Furthermore, the method according to the invention is particularly energy efficient because it does not require any additional signal transmission. If this is permitted by applicable standards, it is conceivable to omit the process described in the introduction, which is both lengthy and energy-intensive because it requires several signal transmission cycles from the RF reader, and thus drastically reduces the power consumption of the near-field communication system.

[0028] Preferably, the presence detection includes measuring the amplitude of the radio frequency supply signal and comparing it with the amplitude value of the radio frequency supply signal in the absence of a radio frequency transponder, which is called the no-load value.

[0029] Advantageously, the presence of an RF transponder is detected when the deviation between the measured value of the amplitude of the RF supply signal and the no-load value is greater than a predetermined deviation threshold.

[0030] As a variation, the presence of an RF transponder can be detected when the ratio between the measured amplitude of the RF supply signal and the no-load value is less than a predetermined ratio threshold.

[0031] According to another variation, the presence of an RF transponder can be detected when the deviation between the measured value of the amplitude of the RF supply signal and the idle value on one hand is less than the ratio between the idle value on the other hand and a predetermined ratio threshold.

[0032] Preferably, the amplitude of the radio frequency supply signal is maintained at or greater than half of the no-load value, even in the presence of a radio frequency transponder.

[0033] In the absence of a radio frequency transponder, the radio frequency supply signal advantageously has a constant amplitude.

[0034] The radio frequency supply signal can have a duration between 250 microseconds and 6 seconds.

[0035] The method according to the invention is advantageously implemented using a passive type of radio frequency transponder.

[0036] Preferably, when the absence of the radio frequency transponder is detected at the end of the step of detecting the presence of the radio frequency transponder, the signal transmission of the radio frequency reader is suspended.

[0037] The method may also include receiving a returned radio frequency signal, which corresponds to a radio frequency supply signal modulated by a radio frequency transponder, and contains authentication information of the radio frequency transponder, which is intended to be used to authorize entry into the motor vehicle.

[0038] The present invention also relates to the use of the method according to the invention, which is implemented after a first transition in response to authentication of a radio frequency transponder from a locked or unlocked state of at least one opening element of a motor vehicle, or from an open or closed state of at least one opening element of a motor vehicle, or from an open or closed state of at least one window of a motor vehicle, wherein a new transition from that state is authorized only if the absence of the radio frequency transponder has been detected at the end of the step of detecting the presence of the radio frequency transponder.

[0039] The present invention also relates to the use of a method according to the invention, which is implemented to detect back-and-forth movement of an RF transponder relative to an RF reader, said movement being associated with a command to transition from a locked or unlocked state of at least one opening element of a motor vehicle, or from an open or closed state of at least one opening element of a motor vehicle, or from an open or closed state of at least one window of a motor vehicle.

[0040] Finally, the present invention covers an RF reader intended for use in a motor vehicle for communication with an RF transponder carried in use by a user located outside the vehicle, the RF reader being intended to communicate with the RF transponder in order to control access to the motor vehicle, and being configured to implement the steps of the method according to the invention. Attached Figure Description

[0041] Further features and advantages of the invention will become clearer upon reading the following description. This description is purely illustrative and should be read with reference to the accompanying drawings, in which:

[0042] [ Figure 1 ] Figure 1 A near-field communication system is schematically illustrated, in which the presence detection method according to the present invention is implemented;

[0043] [ Figure 2 ] Figure 2 The schematic illustrations depict the steps of the method according to the invention; and

[0044] [ Figure 3 ] Figure 3 The amplitude of a radio frequency signal transmitted by a radio frequency reader implementing the method according to the invention as a function of time is schematically illustrated. Detailed Implementation

[0045] refer to Figure 1 First, a description of a near-field communication system 100 is given, in which the presence detection method according to the present invention is implemented.

[0046] System 100 consists of radio frequency transponder 110 and radio frequency reader 120.

[0047] The term "near-field communication" refers to a short-range, high-frequency wireless communication technology (preferably between 5 MHz and 20 MHz, for example, about 14 MHz) that enables information exchange between devices (here, radio frequency transponder 110 and radio frequency reader 120) at distances up to about ten centimeters. Near-field communication preferably follows the standard abbreviated as NFC.

[0048] In use, the RFID reader 120 is installed on motor vehicles ( Figure 1 The interface 10 between the vehicle and its external environment is schematically shown.

[0049] The radio frequency reader 120 specifically includes at least one antenna 121, a matching circuit 122, a signal preprocessing chip 123, and a microcontroller 124, which are connected together in this order.

[0050] The radio frequency antenna 120, referred to as NFC antenna 121, is positioned near the outer surface of the vehicle during use, such as at the door handle or vertical structural pillar between the front and rear doors. NFC antenna 121 is configured to transmit and receive radio frequency signals, particularly near-field communication signals.

[0051] Matching circuit 122 is configured to perform impedance matching between the impedance of NFC antenna 121 and the impedance of circuitry on signal preprocessing chip 123. Matching circuit 122 advantageously includes at least two metal rails extending on a printed circuit board, each metal rail between a corresponding end of NFC antenna 121 and signal preprocessing chip 123.

[0052] The signal preprocessing chip 123 includes a clock at the frequency of the radio frequency signal to be transmitted, used to generate 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 the signal received by the NFC antenna 121 in phase or quadrature with the clock signal to obtain signals I and Q. The signal preprocessing chip 123 also includes at least one analog-to-digital converter for performing time sampling of the signal.

[0053] Signal preprocessing chip 123 is configured to supply the time-sampled signals I and Q to microcontroller 124. Microcontroller 124 is configured to perform analysis of the signals to extract amplitude and / or phase and / or frequency data, and to implement the method described below.

[0054] Amplitude and / or phase and / or frequency data can be encoded to encode authentication information, which can be directly retrieved within the microcontroller 124 or on a remote computer (not shown).

[0055] In use, the radio frequency transponder 110 is carried by the user, particularly a user located outside the vehicle who wishes to enter the vehicle. The radio frequency transponder 110 may take the form of a dedicated badge or "key fob," or be an integral part of a smartphone. The radio frequency transponder 110 includes at least one NFC antenna 111 and a memory 112.

[0056] NFC antenna 111 is configured to perform near-field communication with antenna 121 of RF reader 120. Memory 112 advantageously stores at least one authentication data.

[0057] System 100, including radio frequency transponder 110 and radio frequency reader 120, is a motor vehicle access system configured to authorize entry into the vehicle only to users with authorized radio frequency transponders 110. More specifically, near-field communication system 100 is configured in use to perform near-field communication between radio frequency transponder 110 and a computer forming a vehicle access management system via radio frequency reader 120. The computer is configured to compare an authentication code stored in radio frequency transponder 110 and transmitted via radio frequency reader 120 with an authentication code associated with at least one authorized transponder and stored in the memory of the vehicle access system. When the authentication codes match, the vehicle access management system generates at least one vehicle entry command, preferably a command for locking or unlocking at least one opening element of the vehicle and / or a command for opening or closing at least one opening element of the vehicle and / or a command for opening or closing at least one window of the vehicle. Throughout the text, opening element refers to a front or rear trunk door or tailgate, or a front or rear trunk opening element.

[0058] Advantageously, but not limitingly, the radio frequency transponder 110 is passive. This means that it does not include an energy source that would allow it to generate its own radio frequency signal. It transmits information by modifying the radio frequency signal generated by the radio frequency reader 120. In particular, the radio frequency transponder 110 is then configured to modulate the radio frequency signal supplied by the radio frequency reader 120 in order to encode data such as authentication data. The modulation is advantageously amplitude modulation.

[0059] Now refer to Figure 2 and Figure 3 The steps of the method according to the present invention are described.

[0060] Figure 2 The steps of the method according to the invention implemented in the near-field communication system 100 are illustrated in more detail.

[0061] Figure 3 The evolution of the amplitude of the radio frequency signal emitted by the radio frequency reader 120 as a function of time is illustrated more specifically and schematically.

[0062] In the preparatory steps, and in a manner known per se, the RF reader 120 performs a presence detection known as low-power card detection or LPCD. This presence detection involves transmitting very short, time-interval RF pulses and modifying at least one parameter among the amplitude, phase, and frequency of the RF transponder 110, which indicates the presence of the RF reader 120 in its near field (in fact, a radius of ten centimeters or less). Figure 3In this context, the LPCD pulse corresponds to signal section 31, and the input of the RF transponder 110 into the near field of the RF reader 120 is marked by the modification of the pulse amplitude (the transition from amplitude A1 to amplitude A2).

[0063] In response to the presence detection, the RF reader 120 switches from standby mode to active mode and begins near-field communication with the RF transponder 110.

[0064] In the first step 21 of this communication, the RF reader 120 transmits an RF interrogation signal 32, which constitutes a request for data from the RF transponder 110. As detailed in the description, various modulations can be implemented: amplitude and / or phase and / or frequency modulation. Figure 3 An example of amplitude modulation is illustrated in more detail. As explained in the introduction, and in a manner known per se, amplitude modulation is characterized by its waveform and by the ratio between the maximum peak-to-peak amplitude (encoding bits of a unit value) and the minimum peak-to-peak amplitude (encoding bits of a zero value).

[0065] In the second step 22 of this communication, the RF reader 120 transmits an unmodulated RF supply signal 33. The RF supply signal 33 is intended to be received by the RF transponder 110 and then modulated (in amplitude and / or phase and / or frequency) for the transmission of data requested by the RF interrogation signal, preferably authentication data.

[0066] The radio frequency supply signal 33 is matched with a passive type of radio frequency transponder. However, regardless of whether the radio frequency transponder 110 is passive or active, the radio frequency reader 120 transmits such a radio frequency supply signal after the transmission sequence of the radio frequency interrogation signal. As detailed in the description, the radio frequency supply signal 33 is also referred to as the "FWI frame".

[0067] According to the present invention, step 22 is followed by step 23, which detects the presence of radio frequency transponder 110 by measuring the amplitude of radio frequency supply signal 33 and comparing it with at least one predetermined threshold.

[0068] In other words, the presence detection step 23 includes step 230 of measuring the amplitude of the radio frequency supply signal, step 231 of comparing the quantity as a function of the measured amplitude with a predetermined threshold, and step 232 of finally determining whether the radio frequency transponder 110 is present or absent in the near field of the radio frequency reader 120.

[0069] These steps are implemented by the microcontroller 124 of the RF reader 120.

[0070] Advantageously, the presence detection step 23 begins at the same time as step 22, which transmits the radio frequency supply signal, in order to detect the absence of the radio frequency transponder as quickly as possible. This rapid presence detection is particularly advantageous, especially when using this method to detect one or more back-and-forth movements (one or more tappings) of the radio frequency transponder relative to the radio frequency reader. Such movements can control vehicle functions, particularly those for entering the vehicle, such as those described above (locking, unlocking, opening / closing opening elements, opening / closing windows).

[0071] The radio frequency supply signal 33 has a duration between 250 microseconds and 6 seconds. The predetermined threshold may be approximately half of this duration, for example, between 100 microseconds and 3 seconds.

[0072] Therefore, the method according to the invention proposes to perform presence detection as quickly as possible. This avoids unnecessary signal transmission and associated power consumption by the RF reader 120 when communication standards permit. In particular, it is proposed to cleverly utilize the fact that the RF supply signal 33 is an unmodulated signal, and therefore its amplitude is primarily affected by the presence or absence of the RF transponder 110 in the near field of the RF reader 120. In other words, in the absence of the RF transponder, the RF supply signal 33 has a constant amplitude. Its amplitude may be affected by amplitude modulation controlled by the RF transponder 110, or simply by the presence of the RF transponder in the absence of modulation by the RF transponder 110.

[0073] Advantageously, presence detection involves measuring the amplitude of the radio frequency supply signal and comparing it with the amplitude value of the radio frequency supply signal in the absence of a radio frequency transponder, which is called the no-load value.

[0074] The presence of RF transponder 110 in the near field of RF reader 120 modifies the measured amplitude of the RF supply signal relative to its no-load value in the near field of RF reader 120 where RF transponder 110 is not present. The presence of RF transponder 110 in the near field of RF reader 120 can be detected based on the absolute difference or deviation between the measured amplitude and the no-load amplitude. As a variation, the presence of RF transponder 110 in the near field of RF reader 120 can be detected based on the ratio between the measured amplitude and the no-load amplitude. It is also possible to use the ratio between the deviation and the no-load amplitude. Those skilled in the art will advantageously choose one or more of these criteria as a function of the expected deviation of the no-load amplitude and in order to minimize the false detection rate.

[0075] In all cases, the presence of the radio frequency transponder 110 has a relatively small impact on the amplitude of the radio frequency supply signal. In particular, even in the presence of the radio frequency transponder 110, the amplitude of the radio frequency supply signal 33 remains greater than or equal to half of its no-load value, and even up to 80%.

[0076] 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 of the radio frequency reader 120 is paused, and the latter can even switch back to standby mode. The power consumption of the radio frequency reader 120 is thus minimized, which has a clear advantage in the case of components in motor vehicles.

[0077] On the other hand, when the presence of the radio frequency transponder 110 in the near field of the radio frequency reader 120 is detected in step 23, the method then advantageously includes receiving a returned radio frequency signal, corresponding to the radio frequency supply signal 33 modulated by the radio frequency transponder 110 or corresponding to a radio frequency signal generated by an active transponder. The returned radio frequency signal contains an authentication code of the radio frequency transponder 110, which is intended to be extracted and analyzed by the aforementioned vehicle access management system to authorize entry into the motor vehicle. In particular, authorization or non-authorization is determined by at least one of the following actions: locking or unlocking at least one opening element of the vehicle, opening or closing at least one opening element of the vehicle, or opening or closing at least one window of the vehicle.

[0078] This invention discovers a particularly advantageous application for quickly and almost without signaling confirming that the radio frequency transponder 110 has left the near field of the radio frequency reader 120 after a first transition from a locked or unlocked state of at least one opening element of a motor vehicle (transition from a locked state to an unlocked state, or vice versa), or an open or closed state of at least one opening element of a motor vehicle (transition from an open state to a closed state, or vice versa). This is done only when the absence of the radio frequency transponder 110 has already been detected. In other words, a new transition from the state is only authorized when it has been confirmed that the radio frequency transponder 110 has left the near field of the radio frequency reader 120 since the previous transition. The aim is to avoid untimely state transitions with several consecutive state changes while the radio frequency transponder 110 has not yet left the near field of the radio frequency reader 120.

[0079] Therefore, the present invention proposes a technical solution for detecting the presence of an external NFC device (RF transponder 110) without having to wait for the communication to end.

[0080] One of the objectives is to authorize new communication with the same external NFC device (RF transponder 110) only when the external NFC device has left the near field of the NFC reader (RF reader 120) at the same time, and thus authorize new transitions, particularly the locking or unlocking of at least one unlocking element of the motor vehicle.

Claims

1. A method implemented in a near-field communication system (100) for detecting the presence of an RF transponder (110) by means of an RF reader (120) on a motor vehicle, the RF transponder (110) being carried in use by a user located outside the vehicle, and the RF reader (120) being intended to communicate with the RF transponder (110) to control access to the motor vehicle, the method comprising the following steps implemented by the RF reader: - Transmit (21) an radio frequency interrogation signal (32), which is modulated and forms a request to read data from the radio frequency transponder (110); then - Transmit (22) radio frequency supply signal (33), which is intended to be received by radio frequency transponder (110) and then modulated for transmission of data requested by radio frequency interrogation signal; The method further includes a step (23) of detecting the presence of a radio frequency transponder (110) by measuring the amplitude of a radio frequency supply signal and comparing it with at least one predetermined threshold.

2. The method according to claim 1, wherein, Presence detection (23) includes measuring the amplitude of the radio frequency supply signal (33) and comparing it with the amplitude value of the radio frequency supply signal in the absence of a radio frequency transponder, which is referred to as the no-load value.

3. The method according to claim 2, wherein, The presence of an RF transponder (110) is detected when the deviation between the measured value of the amplitude of the RF supply signal (33) and the no-load value is greater than a predetermined deviation threshold.

4. The method according to claim 2, wherein, The presence of an RF transponder (110) is detected when the ratio between the measured amplitude of the RF supply signal and the no-load value is less than a predetermined ratio threshold.

5. The method according to claim 2, wherein, The presence of an RF transponder (110) is detected when the deviation between the measured value of the amplitude of the RF supply signal (33) on one side and the empty value on the other side is less than a predetermined ratio threshold.

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

7. The method according to any one of claims 1 to 6, wherein, In the absence of a radio frequency transponder (110), the radio frequency supply signal (33) has a constant amplitude.

8. The method according to claim 7, wherein, The radio frequency supply signal (33) has a duration between 250 microseconds and 6 seconds.

9. The method according to any one of claims 1 to 8, wherein, The method is implemented using a passive type of radio frequency transponder (110).

10. The method according to any one of claims 1 to 9, wherein, When the absence of the radio frequency transponder (110) is detected at the end of step (23) of detecting the presence of the radio frequency transponder (110), the signal transmission of the radio frequency reader (120) is paused.

11. The method according to any one of claims 1 to 10, wherein, The method further includes receiving a returned radio frequency signal corresponding to a radio frequency supply signal (33) modulated by a radio frequency transponder (110), and includes authentication information of the radio frequency transponder (110) intended for use in authorizing access to the motor vehicle.

12. The use of the method according to any one of claims 1 to 11, wherein the method is implemented after a first transition from a locked or unlocked state of at least one opening element of the motor vehicle, or from an open or closed state of at least one opening element of the motor vehicle, or from an open or closed state of at least one window of the motor vehicle, in response to the authentication of the radio frequency transponder (110), wherein a new transition from that state is authorized only if the absence of the radio frequency transponder (110) has been detected at the end of step (23) of detecting the presence of the radio frequency transponder (110).

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

14. A radio frequency reader (120) intended for use in a motor vehicle to communicate with a radio frequency transponder (110) carried in use by a user located outside the vehicle, the radio frequency reader (120) being intended to communicate with the radio frequency transponder (110) to control access to the motor vehicle, and configured to implement the steps of the method according to any one of claims 1 to 11.

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