Communication method of a pressure monitoring system

Abstract

The invention relates to a method for communicating a monitoring system comprising a wheel unit (16a, 16b, 16c, 16d) intended to communicate with an electronic central unit (12) by means of low energy Bluetooth® communication to establish a two-way communication between the wheel unit and the electronic central unit, the wheel unit being able to transmit periodic signals of short durations to search for a connection with the electronic central unit, the electronic central unit being able to acknowledge the periodic signals by transmitting a frame of radiofrequency signals to the wheel unit.The method comprises a step of automatically adapting the radiofrequency transmission power of the wheel unit when the wheel unit is in “presence state” mode, during which the wheel unit transmits signals of variable radiofrequency power so as to determine a minimum radiofrequency power from which the electronic central unit is capable of acknowledging at least one signal by transmitting a frame of radiofrequency signals to the wheel unit. Figure for the abstract: Figure 1.

Description

Title of the invention: Communication method of a pressure monitoring system Technical field

[0001] The present invention relates to a communication method, and in particular to a pressure monitoring system intended to measure the pressure in a tire of a motor vehicle. Prior art

[0002] Automotive safety legislation requires manufacturers to provide means of monitoring tire pressure.

[0003] For this purpose, several pressure monitoring systems have been developed and put on the market in recent years.

[0004] As is known per se, these pressure monitoring systems, commonly referred to as "TPMS" (Tire Pressure Monitoring System), typically comprise at least one wheel unit (comprising at least one pressure sensor) arranged inside the chamber defined by the tire and the rim of the wheel of a vehicle, and remote calculation means communicating with this wheel unit by radio frequencies, these calculation means generally being located in the electronic computer of an on-board electronic central unit of the vehicle (commonly referred to as "ECU" for Electronic Control Unit in English).

[0005] The wheel unit may be fixed inside the tread of the tire, or on the inflation valve of this tire, which is mounted inside an orifice formed on the rim.

[0006] The wheel unit is typically powered by a non-rechargeable battery which may have a lifespan of ten years, for example.

[0007] Communication between the wheel unit and the ECU is performed via a radio frequency communication network. The wheel unit comprises a first transmitting / receiving device and the ECU comprises a second transmitting / receiving device.

[0008] A radio frequency frame, generally called an “RF frame”, is transmitted by the wheel unit and received by the ECU in order to transmit, in particular, wheel pressure parameters.

[0009] Radio frequency communication is carried out using low energy Bluetooth® (commonly referred to as “BLE” for Bluetooth® Low Energy in English) allowing the bidirectional exchange of data over short distances using UHF (ultrahigh frequency) radio waves on the 2.4 GHz frequency band.

[0010] The advantage of Bluetooth® Low Energy over “classic” Bluetooth® is that it allows sending small amounts of data while consuming very little energy.

[0011] It is possible to use low energy Bluetooth® in "presence state" (or "advertising" in English) mode in which the wheel unit emits periodic signals. The ECU analyzes its environment by performing scans (called "scan-request" in English) for very short periods of time to check whether these signals are transmitted by the wheel unit. In other words, the wheel unit transmits radio frequency signals. The ECU confirms good reception or acknowledges the radio frequency signals by returning a radio frequency frame to the wheel unit. A communication search is in fact carried out between the wheel unit and the ECU.

[0012] It is also possible to use low energy Bluetooth® in “connected mode” in which communication between the wheel unit and the ECU is well established after the “presence state” mode. This connected mode allows the transmission of pressure data by radio frequency frame.

[0013] This “connected mode” is very energy-intensive. There are mechanisms called “acknowledgment” which optimizes the energy consumption of the wheel unit during communication in Bluetooth® low energy based on the receiver feedback loop.

[0014] However, this type of mechanism does not exist for the “presence state” mode while it is necessary to reduce or optimize the energy consumption of the wheel unit regardless of the mode used in order to extend the battery life. Statement of the invention

[0015] The invention solves this problem by proposing a method for communicating a monitoring system in low energy Bluetooth® making it possible to optimize the radiofrequency transmission power of the wheel unit when it communicates in “presence state” mode, in order to limit its electrical consumption.

[0016] The invention relates to a method for communicating a monitoring system comprising a wheel unit capable of carrying out measurements in a vehicle and an electronic central unit remote from the wheel unit. The wheel unit comprises a first radio frequency transmitter / receiver device intended to communicate with a second radio frequency transmitter / receiver device provided on the electronic central unit by means of Bluetooth® low energy communication to transmit the measurements to the electronic central unit and establish a two-way communication between the wheel unit and the electronic central unit. The wheel unit is capable of operating in a “presence state” communication mode in which it transmits periodic signals of short durations to search for a connection with the electronic central unit. The electronic central unit is able to acknowledge the periodic signals by transmitting a frame of radio frequency signals to the wheel unit.

[0017] According to the invention, the method comprises a step of automatically adapting the radiofrequency transmission power of the wheel unit when the wheel unit is in “presence state” mode, during which the wheel unit transmits signals of variable radiofrequency power so as to determine a minimum radiofrequency power from which the electronic central unit is capable of acknowledging at least one radiofrequency signal by transmitting a frame of radiofrequency signals to the wheel unit.

[0018] The invention thus provides a method for communicating a low energy Bluetooth® monitoring system making it possible to optimize the radiofrequency transmission power of the wheel unit when it communicates in “presence state” mode, in order to limit its electrical consumption and extend the life of the battery (or cell).

[0019] The radiofrequency transmission power of the wheel unit adapts dynamically according to the reception context.

[0020] It is possible to regularly retrieve the radiofrequency power data delivered by the wheel unit and obtained after each power optimization sequence, for processing in a set of networked servers ("Cloud" in English) via the electronic central unit.

[0021] This information is very useful because it provides information on the radio frequency powers actually used and optimized to ensure communication in vehicles throughout the lifetime of the product and in all real operating situations (type of vehicle, type of wheels, weather scenarios, urban scenarios, highways, countryside, etc.).

[0022] This data makes it possible to optimize radio frequency architectures in future low frequency Bluetooth® wheel unit projects.

[0023] Indeed, from the actual useful power, it is possible to adjust the type of battery, the dimensions of the antenna and indirectly the dimensions of the box housing these elements.

[0024] Alternatively, a maximum radio frequency power threshold is defined. The wheel unit begins by transmitting a radio frequency signal having the maximum radio frequency power threshold, then transmits radio frequency signals by decreasing their power until the electronic central unit can no longer decode the radio frequency signal. The power of the last acknowledged radio frequency signal is defined as the minimum radio frequency power by the wheel unit.

[0025] Alternatively, a minimum radio frequency power threshold is defined. The wheel unit begins by transmitting a radio frequency signal having the radio power threshold minimum frequency, then emits radio frequency signals increasing their power until the electronic central unit acknowledges a first signal. The power of the first acknowledged radio frequency signal is defined as the minimum radio frequency power by the wheel unit.

[0026] These self-calibration modes have the advantage of being simple and quick to implement, which limits energy consumption. According to these two variants, only the wheel unit performs an action to determine the minimum radiofrequency power.

[0027] Alternatively, several series of radiofrequency signal transmissions are carried out, including a first series with radiofrequency signals having a predefined maximum radiofrequency power. Each subsequent series of transmissions is carried out with radiofrequency signals having a decreasing power level. For each series of transmissions, several radiofrequency signals of the same power are transmitted per wheel revolution and according to equidistant angular positions on the wheel. The wheel unit determines a successful reception rate by a statistical calculation and for several wheel revolutions. The minimum power level is determined from a predefined acceptable successful reception rate.

[0028] Alternatively, several series of radiofrequency signal transmissions are carried out, including a first series with radiofrequency signals having a predefined minimum radiofrequency power. Each subsequent series of transmissions is carried out with radiofrequency signals having an increasing power level. For each series of transmissions, several radiofrequency signals of the same power are transmitted per wheel revolution and according to equidistant angular positions on the wheel. The wheel unit determines a successful reception rate by a statistical calculation and for several wheel revolutions. The minimum power level is determined from a predefined acceptable successful reception rate.

[0029] These variants make it possible to take into account non-reception zones which block the radiofrequency signal and for which the electronic central unit cannot respond. These non-reception zones are due to elements of the vehicle such as the shock absorber or bodywork elements, for example.

[0030] Alternatively, the method comprises a calculation by dichotomy. A minimum power threshold Smin and a maximum power threshold Smax are defined. An average power level is defined according to the relationship Pmoy = (Smin+Smax) / 2. The wheel unit transmits a signal with the average power level Pmoy and checks its acceptance by the electronic central unit. The average power level Pmoy is considered as the new maximum power threshold Smax if the electronic central unit accepts the signal transmitted with the average power level Pmoy. The average power level is considered as the new minimum power threshold Smin if the electronic central unit The electronics does not accept the signal transmitted with the average power level Pmoy. The minimum radio frequency power is determined when Smin and Smax are equal.

[0031] This variant allows convergence by dichotomy, which improves the accuracy of the minimum power level.

[0032] Alternatively, a maximum radio frequency power threshold is defined. The wheel unit begins by transmitting a radio frequency signal having the maximum radio frequency power threshold and then transmits radio frequency signals by decreasing their power until the electronic central unit can no longer decode the signal. The electronic central unit deactivates its ability to acknowledge the radio frequency signals to signal to the wheel unit that the power of the last radio frequency signal received must be defined as the minimum radio frequency power.

[0033] Alternatively, a minimum radio frequency power threshold is defined. The wheel unit begins by transmitting a radio frequency signal having the minimum radio frequency power threshold and then transmits radio frequency signals by increasing their power until the electronic central unit acknowledges the signal. The electronic central unit activates its ability to accept signals to signal to the wheel unit that the power of the first acknowledged radio frequency signal is to be defined as the minimum radio frequency power.

[0034] These last two variants make it possible to increase the reliability of the process.

[0035] The wheel unit is mounted on a wheel of the vehicle and intended to measure the pressure inside a tire.

[0036] The invention also relates to a vehicle comprising at least one monitoring system implementing the communication method as defined previously. Brief description of the drawings

[0037] [Fig.l] [Fig.l] is a schematic top view of a motor vehicle which comprises a TPMS monitoring system capable of implementing a communication method, according to the invention. Description of the embodiments

[0038] [Fig.l] illustrates a motor vehicle 10 which comprises an electronic central unit 12 and four wheels 14a, 14b, 14c, 14d which are each equipped with a wheel unit 16a, 16b, 16c, 16d respectively.

[0039] In order not to complicate the description, only one wheel unit 16a is described below, the four wheel units 16a, 16b, 16c, 16d having a similar design and operation.

[0040] The electronic central unit 12 of the motor vehicle 10, known as “ECU” for “Electronic Control Unit” in English, comprises a cal- electronic controller and memory (not shown).

[0041] Also, the electronic central unit 12 comprises a radio frequency receiver 20.

[0042] The wheel unit 16a comprises a housing which contains a computer 22, a battery 24 and a memory 26.

[0043] In addition, the wheel unit 16a comprises a set of sensors 30 dedicated to measuring operating parameters of the wheel unit 16a.

[0044] This set of sensors 30 comprises a first temperature sensor capable of measuring a temperature parameter T, a second accelerometer-type sensor capable of measuring the acceleration undergone by the wheel unit 16a, a third pressure sensor capable of measuring the inflation pressure of the tire of the associated wheel 14a.

[0045] The measurements taken by this set of sensors 30 can be transmitted to the electronic central unit 12 of the motor vehicle 10 via messages emitted by the transmitter 28 of the associated wheel unit 16a.

[0046] The invention relates to a method of communication of a pressure monitoring system of the "TPMS" (Tire Pressure Monitoring System) type, and which typically comprises at least one wheel unit 16a arranged inside the chamber defined by the tire and the rim of the wheel 14a of a vehicle, and remote calculation means communicating with this wheel unit 16a by mono or bidirectional radio frequencies, these calculation means being generally located in the electronic central unit 12 on board the vehicle (commonly referred to as "ECU" Electronic Control Unit in English), the electronic central unit 12 and the wheel unit 16a being duly programmed to implement the method according to the invention.

[0047] The wheel unit 16a can be fixed inside the tread of the tire, or on the inflation valve of this tire, which is mounted inside an orifice formed on the rim of a wheel 14a of the vehicle 10.

[0048] The wheel unit 16a of the tire is typically powered by a non-rechargeable battery 24 which may have a lifespan of ten years, for example.

[0049] Communication between the wheel unit 16a and the ECU 12 is performed via a radio frequency communication network. The wheel unit 16a comprises a first radio frequency transmitter / receiver device 28 and the ECU 12 comprises a second radio frequency transmitter / receiver device 20.

[0050] A radio frequency frame, generally called an “RF frame” is transmitted by the wheel unit 16a and received by the ECU 12 in order to transmit pressure parameters of the wheel 14a.

[0051] Radio frequency communication is carried out using low energy Bluetooth® (commonly referred to as “BLE” Bluetooth® Low Energy in English) allowing short-distance two-way data exchange using UHF radio waves in the 2.4 GHz frequency band.

[0052] The wheel unit 16a can operate according to two communication modes, including a “presence state” mode in which short-duration periodic signals are transmitted by the wheel unit 16a to search for a connection with the electronic central unit 12 and a “connected” mode in which a stable connection is established between the wheel unit 16a and the electronic central unit 12 for the transmission of pressure measurements.

[0053] The “presence state” (or “advertising” in English) mode is used first to try to establish a radiofrequency connection with the electronic central unit 12 and switch to “connected” mode which allows pressure information to be exchanged by radiofrequency frame between the wheel unit 16a and the electronic central unit 12. There is a feedback loop in “connected” mode which allows the power of the transmission signal to be adapted automatically. This feedback loop is provided by the Bluetooth® low energy BLE 5.0 standard.

[0054] For the invention, the “presence state” communication mode is used. The “connected” mode is not used.

[0055] In “presence state” communication mode, a two-way communication is established between the wheel unit 16a and the electronic central unit 12. The electronic central unit 12 acknowledges the periodic signals by transmitting a frame of radiofrequency signals to the wheel unit 16a. In other words, the electronic central unit 12 confirms the correct reception of the received signal by returning a specific radiofrequency frame to the wheel unit 16a. This operation of transmitting frames of radiofrequency signals is commonly referred to by the English term “scan-request”.

[0056] The transmission power of the wheel unit 16a is fixed and high, to ensure a good radiofrequency reception balance by the electronic central unit 12.

[0057] When the electronic central unit 12 of the vehicle 10 is activated, the electronic central unit 12 of the monitoring system analyzes or scans its environment to search for the connection with the wheel unit 16a. When the electronic central unit 12 picks up a signal transmitted by the wheel unit 16a, the electronic central unit 12 accepts or acknowledges this signal and notifies it to the wheel unit 16a.

[0058] The vehicle 10 can be moving or stationary.

[0059] According to the invention, the communication method of the pressure monitoring system comprises a step of automatically adapting the radiofrequency transmission power of the wheel unit 16a when the wheel unit 16a is in “presence state” mode, during which the wheel unit 16a transmits signals of variable radiofrequency power so as to determine a minimum radiofrequency power from which the electronic central unit 12 is capable of acknowledging at least one signal in transmitting a frame of radio frequency signals to the wheel unit 16a.

[0060] According to a variant, the wheel unit 16a operates in self-calibration mode in which the wheel unit 16a checks whether the electronic central unit 12 has been able to read a message sent by the wheel unit 16a. The wheel unit 16a operates autonomously with minimal participation from the electronic central unit 12 which simply acknowledges or not the message sent by the wheel unit 16a.

[0061] According to one variant, a maximum radiofrequency power threshold is defined. The wheel unit 16a begins by transmitting a radio frequency signal having this maximum radio frequency power threshold and then transmits other radio frequency signals by progressively decreasing their power until the electronic central unit 12 can no longer decode the signal, meaning that the signal power is not high enough to establish radio frequency communication. The power of the last acknowledged radio frequency signal is then defined as the minimum radio frequency power by the wheel unit 16a.

[0062] According to a variant, before the step of determining the maximum radiofrequency power threshold, it is first verified that a known electronic central unit 12 responds to the signals emitted at the defined nominal power.

[0063] The maximum radio frequency power threshold is +4dBm, for example.

[0064] The wheel unit 16a operating in "presence state" mode begins by emitting a radio frequency signal with a radio frequency power of +4dBm. If the signal is acknowledged by the electronic central unit 12, the power level of the next radio frequency signal is lowered and is therefore less than +4dBm.

[0065] The process stops when the electronic central unit 12 no longer acknowledges a transmitted signal. The wheel unit 16a then considers the previous signal as the one having the minimum radio frequency power and records this minimum radio frequency power value.

[0066] According to another variant, a minimum radiofrequency power threshold is defined.

[0067] During the automatic power adaptation step, the wheel unit 16a begins by emitting a radiofrequency signal having the minimum radiofrequency power threshold and then emitting radiofrequency signals by increasing their power until the electronic central unit 12 acknowledges a first signal.

[0068] The power of the first acknowledged radio frequency signal is defined as the minimum radio frequency power by the wheel unit 16a.

[0069] According to a variant, before the step of determining the minimum radiofrequency power threshold, it is first verified that a known electronic central unit responds to the signals emitted at the defined nominal power.

[0070] The minimum radio frequency power threshold is -4dBm, for example.

[0071] The wheel unit 16a operating in “presence state” mode begins by transmitting a radio frequency signal with a radio frequency power of -4dBm. If the signal is not acknowledged by the electronic central unit 12, the power level of the next radio frequency signal is incremented and is therefore greater than -4dBm.

[0072] The process stops when the electronic central unit 12 acknowledges a first transmitted signal. The wheel unit 16a then considers this first signal as the one having the minimum radiofrequency power and records this minimum radiofrequency power value.

[0073] According to another variant, several emission series are carried out, including a first series with radiofrequency signals having a predefined maximum radiofrequency power. This maximum radiofrequency power is +4dBm, for example.

[0074] Each subsequent transmission series is carried out with radiofrequency signals having a progressively decreasing power level. For each transmission series, several radiofrequency signals of the same power are transmitted per wheel revolution and at equidistant angles on the wheel 14a.

[0075] For example, three radio frequency signals can be emitted with identical content on one wheel rotation 14a, with the same power level, every 120°.

[0076] The wheel unit 16a determines a successful reception rate by statistical calculation and for several revolutions of the wheel 14a. The signal power level is then decreased for the following series of measurements.

[0077] The minimum power level is determined from a predefined acceptable successful reception rate.

[0078] If with the maximum radio frequency power of +4dBm, for example, the electronic central unit 12 receives the majority of the signals, the success rate is high and above the predefined acceptable successful reception rate.

[0079] A new series of radiofrequency signal transmissions is then carried out with a radiofrequency power level lower than +4dBm.

[0080] The radio frequency power is reduced in subsequent series until the success rate is no longer acceptable. The radio frequency power of the last series that led to an acceptable successful reception rate is defined as the minimum radio frequency power.

[0081] Indeed, if the last series of signals tested leads to a success rate below the predefined acceptable successful reception rate, it is the penultimate series of signals tested and having led to a success rate above the predefined acceptable successful reception rate, which is considered to define the minimum power level.

[0082] This variant makes it possible to take into account non-reception zones (called “black spots” in English) which block the radiofrequency signal and for which the central unit electronics 12 cannot respond. These areas of no reception are due to vehicle elements such as the shock absorber or bodywork elements, for example.

[0083] Alternatively, in a similar manner, several transmission series are carried out, including a first series with radiofrequency signals having a predefined minimum radiofrequency power. Each subsequent transmission series is carried out with radiofrequency signals having a progressively increasing power level. For each transmission series, several radiofrequency signals of the same power are transmitted per revolution of the wheel 14a and at equidistant angles on the wheel 14a. For example, three radiofrequency signals can be transmitted with identical content on one revolution of the wheel 14a, with the same power level, every 120°. The wheel unit 16a determines a successful reception rate by statistical calculation and for several revolutions of the wheel 14a. The wheel unit 16a determines a successful reception rate by statistical calculation and for several revolutions of the wheel 14a.The minimum power level is determined from a predefined acceptable successful reception rate.

[0084] According to another variant, the method comprises a calculation by dichotomy. A minimum power threshold Smin and a maximum power threshold Smax are defined. An average power level is also defined according to the relation Pmoy = (Smin+Smax) / 2.

[0085] The wheel unit 16a emits a signal with the average power level Pmoy and checks whether it is acknowledged by the electronic central unit 12.

[0086] The average power level Pmoy is considered as the new maximum power threshold Smax if the electronic central unit 12 acknowledges the signal transmitted with the average power level Pmoy.

[0087] Otherwise, the average power level is considered as the new minimum power threshold Smin if the electronic central unit 12 does not acknowledge the signal transmitted with the average power level Pmoy.

[0088] This algorithm is performed as long as Smin and Smax are different. The minimum radio frequency power is determined when Smin and Smax are equal. The last maximum power threshold Smax tested is thus considered as the minimum radio frequency power.

[0089] The preceding variants are applicable when the wheel unit 16a operates in a self-calibration (or automatic) mode, the electronic central unit 12 simply acknowledging or not acknowledging the signals.

[0090] According to another variant, the electronic central unit 12 takes control to define the minimum radiofrequency power.

[0091] The wheel unit 16a emits a radio frequency signal with maximum power, and decreases the power until the electronic central unit 12 considers that the received field level is within the acceptable low limit.

[0092] In other words, a maximum radiofrequency power threshold is defined. The wheel unit 16a begins by emitting a radiofrequency signal having the maximum radiofrequency power threshold and then emits radiofrequency signals while decreasing their power until the electronic central unit 12 can no longer decode or acknowledge the signal.

[0093] The power of the last signal acknowledged by the electronic central unit 12 is considered to be the minimum radio frequency power. The electronic central unit 12 deactivates its ability to acknowledge signals to indicate to the wheel unit 16a that the power of the last radio frequency signal received must be defined as the minimum radio frequency power.

[0094] The electronic central unit 12 now ignores the received radio frequency frames. The wheel unit 16a understands that the signal is not received by the electronic central unit 12 and thus sets the minimum acceptable power level to the previous functional value.

[0095] According to another variant, instead of starting from a high power limit, the wheel unit 16a begins by emitting signals in the low power limit until it detects an acknowledgment by the electronic central unit 12.

[0096] Indeed, a minimum radiofrequency power threshold is defined. The wheel unit 16a begins by emitting a radiofrequency signal having the minimum radiofrequency power threshold and then emits subsequent radiofrequency signals, increasing their power each time until the electronic central unit 12 acknowledges the signal.

[0097] The electronic central unit 12 then activates its signal acknowledgment capability to signal to the wheel unit 16a that the power of the first acknowledged radiofrequency signal must be defined as the minimum radiofrequency power.

[0098] Alternatively, a waiting time is provided after a signal not acknowledged by the electronic central unit 12.

[0099] The previously described variants are given for the case of a wheel unit 16a as an example.

[0100] Alternatively, it is possible to regularly retrieve the radiofrequency power data delivered by the wheel unit 16a and obtained after each power optimization sequence, for processing in a set of networked servers (“Cloud” in English) via the electronic central unit 12.

[0101] This information is very useful because it provides information on the radio frequency powers actually used and optimized to ensure communication in vehicles throughout the lifetime of the product and in all real operating situations (type of vehicle, type of wheels, weather scenarios, urban scenarios, highways, countryside, etc.).

[0102] This data makes it possible to optimize radio frequency architectures in future low frequency Bluetooth® wheel unit projects.

[0103] Indeed, from the actual useful power, it is possible to adjust the type of battery, the dimensions of the antenna and indirectly the dimensions of the box housing these elements.

Claims

Claims

1. A method of communicating a monitoring system comprising a wheel unit (16a, 16b, 16c, 16d) capable of performing measurements in a vehicle (10) and an electronic central unit (12) remote from the wheel unit (16a, 16b, 16c, 16d), the wheel unit (16a, 16b, 16c, 16d) comprising a first radio frequency transmitter / receiver device (28) intended to communicate with a second radio frequency transmitter / receiver device (20) provided in the electronic central unit (12) by means of Bluetooth® communication allowing the bidirectional exchange of data at short distance by using low-energy ultra-high frequency radio waves to transmit the measurements to the electronic central unit (12) and establish a bidirectional communication between the wheel unit (16a, 16b, 16c, 16d) and the unit electronic control unit (12), wheel unit (16a, 16b, 16c,16d) being capable of operating according to a “presence state” communication mode in which it transmits periodic signals of short durations to search for a connection with the electronic central unit (12), the electronic central unit (12) being capable of acknowledging the periodic signals by transmitting a frame of radiofrequency signals to the wheel unit (16a, 16b, 16c, 16d), characterized in that it comprises a step of automatically adapting the radiofrequency transmission power of the wheel unit (16a, 16b, 16c, 16d) when the wheel unit (16a, 16b, 16c, 16d) is in “presence state” mode, during which the wheel unit (16a, 16b, 16c, 16d) transmits signals of variable radiofrequency power so as to determine a minimum radiofrequency power from which the electronic central unit (12) is capable of acknowledging at least one signal by transmitting a frame of radio frequency signals) to the wheel unit (16a, 16b, 16c, 16d).,

2. Method according to claim 1, characterized in that a maximum radio frequency power threshold is defined, the wheel unit (16a, 16b, 16c, 16d) starting by transmitting a radio frequency signal having the maximum radio frequency power threshold, then transmitting radio frequency signals by decreasing their power until the electronic central unit (12) can no longer decode the radio frequency signal, the power of the last acknowledged radio frequency signal being defined as minimum radio frequency power by the wheel unit (16a, 16b, 16c, 16d).

3. Method according to claim 1, characterized in that a minimum radio frequency power threshold is defined, the wheel unit (16a, 16b, 16c, 16d) starting by transmitting a radio frequency signal having the minimum radio frequency power threshold, then transmitting radio frequency signals by increasing their power until the electronic central unit (12) acknowledges a first radio frequency signal, the power of the first acknowledged radio frequency signal being defined as minimum radio frequency power by the wheel unit (16a, 16b, 16c, 16d).

4. Method according to one of claims 1 or 2, characterized in that several series of radio frequency signal transmissions are carried out, including a first series with radio frequency signals having a predefined maximum radio frequency power, each subsequent series of transmissions being carried out with radio frequency signals having a decreasing power level, for each series of transmissions, several radio frequency signals of the same power being transmitted per wheel revolution (14a, 14b, 14c, 14d) and according to equidistant angular positions on the wheel (14a, 14b, 14c, 14d), the wheel unit (16a, 16b, 16c, 16d) determining a successful reception rate by a statistical calculation and for several wheel revolutions (14a, 14b, 14c, 14d), the minimum power level being determined from a predefined acceptable successful reception rate.

5. Method according to one of claims 1 or 3, characterized in that several series of transmission of radiofrequency signals are carried out, including a first series with radiofrequency signals having a predefined minimum radiofrequency power, each subsequent series of transmission being carried out with radiofrequency signals having an increasing power level, for each series of transmission, several radiofrequency signals of the same power being transmitted per wheel revolution (14a, 14b, 14c, 14d) and according to equidistant angular positions on the wheel (14a, 14b, 14c, 14d), the wheel unit (16a, 16b, 16c, 16d) determining a successful reception rate by a statistical calculation and for several wheel revolutions (14a, 14b, 14c, 14d), the minimum power level being determined from a predefined acceptable successful reception rate.

6. Method according to claim 1, characterized in that it comprises a calculation by dichotomy, a minimum power threshold Smin and a maximum power threshold Smax being defined, an average power level being defined according to the relation Pmoy = (Smin+Smax) / 2, the wheel unit (16a, 16b, 16c, 16d) emitting a signal with the average power level Pmoy and controlling its acceptance by the electronic central unit (12), the average power level Pmoy being considered as the new maximum power threshold Smax if the electronic central unit (12) accepts the signal emitted with the average power level Pmoy, the average power level being considered as the new minimum power threshold Smin if the electronic central unit (12) does not accept the signal emitted with the average power level Pmoy, the minimum radiofrequency power being determined when Smin and Smax are equal.

7. A method according to claim 1, characterized in that a maximum radio frequency power threshold is defined, the wheel unit (16a, 16b, 16c, 16d) starting by transmitting a radio frequency signal having the maximum radio frequency power threshold and then transmitting radio frequency signals by decreasing their power until the electronic central unit (12) can no longer decode the signal, the electronic central unit (12) deactivating its ability to acknowledge the signals to indicate to the wheel unit (16a, 16b, 16c, 16d) that the power of the last radio frequency signal received must be defined as the minimum radio frequency power.

8. A method according to claim 1, characterized in that a minimum radio frequency power threshold is defined, the wheel unit (16a, 16b, 16c, 16d) starting by transmitting a radio frequency signal having the minimum radio frequency power threshold and then transmitting radio frequency signals by increasing their power until the electronic central unit (12) acknowledges the radio frequency signal, the electronic central unit (12) activating its ability to acknowledge the signals to signal to the wheel unit (16a, 16b, 16c, 16d) that the power of the first acknowledged radio frequency signal must be defined as the minimum radio frequency power.

9. A method according to any one of claims 1 to 7, characterized in that the wheel unit (16a, 16b, 16c, 16d) is mounted on a wheel (14a) of a vehicle (10) and intended to monitor the pressure inside a tire.

10. Vehicle (10) characterized in that it comprises at least one monitoring system implementing the communication method as defined according to any one of claims 1 to 9.

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