METHOD FOR CALIBRATING A RADIO-BASED KEYLESS ACCESS SYSTEM OF A MOTOR VEHICLE, ACCESS SYSTEM AND MOTOR VEHICLE

Description

[0001] The invention relates to a method for calibrating a radio-based keyless access system of a motor vehicle, a corresponding radio-based keyless access system and a motor vehicle with such an access system.

[0002] Radio-based keyless entry systems for motor vehicles are already known. Such systems allow a user, for example, to automatically unlock a motor vehicle when a signal transmitter or identification transmitter (ID transmitter) carried by the user is brought close to the motor vehicle. While this signal transmitter can be integrated into a key for the motor vehicle, the term "keyless" in the context of the present invention refers to the fact that the user does not have to manually operate the key or signal transmitter to unlock the motor vehicle. In the corresponding technical field, such access systems are also referred to as keyless access systems, or Kessy systems for short. These access systems are intended to provide or execute a respective functionality, such as automatically unlocking the motor vehicle, at a predetermined distance from the motor vehicle.In practical application, however, the problem can be observed that the respective functionality is actually triggered or executed at different distances from the vehicle depending on the environment. This is undesirable for safety reasons and with regard to the desired consistency or predictability of the access system's behavior.

[0003] DE 10 2015 226 150 A1 already discloses a method for calibrating a radio device for a vehicle locking system comprising multiple antennas. First signals are transmitted by a first antenna provided at a first location of the vehicle and received by the first and / or a second antenna of the vehicle. Second signals are transmitted from a second antenna provided at a second location of the vehicle, which are received by several of the antennas. Based on reception values ​​for the first and second signals, data of a radio environment model for the vehicle is created.

[0004] DE 10 2008 012 882 A1 describes an electrical circuit for access control and for an immobilizer of a vehicle. A first antenna is provided for transmitting a signal when an ID transmitter for the vehicle's access control is identified. Furthermore, a second antenna is provided for transmitting a signal when an ID transmitter for the vehicle's immobilizer is identified. A receiver device is provided for receiving a signal received by an antenna for identifying an ID transmitter for the immobilizer. A first antenna driver for driving the first antenna and the receiver device are integrated together in a semiconductor chip.

[0005] US 2017 / 0 026 910 A1 relates to a passive entry and start system for a vehicle. This system comprises a portable device with a transceiver, a vehicle-based transceiver, and a control unit. The vehicle-based transceiver transmits an interrogation signal, in response to which the transceiver of the portable device transmits a response signal. The control unit determines a distance of the portable device relative to the vehicle-based transceiver as a function of the propagation time of the interrogation and response signals. Depending on the determined distance, the control unit controls a measurement duty cycle of the interrogation signal. A larger duty cycle can be used the closer the portable device is to the vehicle.

[0006] DE 10 2006 008 141 presents a method for operating a radio-based identification system with a central unit and at least one identification transmitter. The central unit generates an electric field with a first piece of identification information, which the identification transmitter detects and generates a second piece of identification information and sends it to the central unit. A time period, related to the transmission of the first piece of identification information, is predetermined in terms of duration and position, and the second piece of identification information is only accepted within this predetermined time period. The time period is preferably variable and is varied during operation.

[0007] DE 10 2015 109 275 A1 relates to a passive access system for a motor vehicle in which the distance between the vehicle key and the vehicle is determined. The proposed method involves measuring the distance by evaluating the received signal strength indicator (RSSI), the time of arrival (TOA), and / or the angle of arrival (AOA).

[0008] DE 102 12 648 A1 relates to an identification system for verifying authorization for access to a motor vehicle. The distance between the vehicle key and the vehicle is determined by measuring the signal propagation time of the response signal or the signal propagation time between the transmission of the query signal and the reception of the response signal. This can be done using time measurement or phase measurement.

[0009] DE 10 2005 058 041 A1 discloses a method for operating an access security system for a motor vehicle. A signal is sent from the vehicle to the vehicle key. The vehicle key generates a second signal, into which the information about the reception strength is inserted, and sends this second signal back to the vehicle via the wireless connection. The reception strength of the first signal is compared with the reception strength of the second signal, and access authorization is determined if the two reception strengths differ by less than a predetermined value.

[0010] DE 10 2016 113 320 A1 shows a method for a radio-based keyless access system for a motor vehicle, in which an LF field is emitted into an environment of the motor vehicle by means of a first transmitting device of the motor vehicle, and based on a field strength of the LF field measured by means of a signal transmitter separate from the motor vehicle, it is automatically determined whether the signal transmitter is located at most a predetermined distance from the motor vehicle.

[0011] DE 11 2014 006 819 T5 describes a passive entry system for a passenger car with a low-frequency transceiver for transmitting a low-frequency signal, an ultra-wideband transceiver, and an evaluation device. The evaluation device has a calculation unit for deriving a first distance measurement of a key, which is encoded in a first received ultra-wideband signal and derived from a signal strength of the low-frequency signal, for deriving a second distance measurement of the key from a reception time of a second received ultra-wideband signal, for comparing the first distance measurement with the second distance measurement, and for determining a distance discrepancy between the first distance measurement and the second distance measurement.

[0012] The object of the present invention is to improve the accuracy of a keyless entry system of a motor vehicle.

[0013] This object is achieved according to the invention by the subject matter of the independent patent claims. Advantageous embodiments and further developments of the present invention are specified in the dependent patent claims, in the description, and in the drawings.

[0014] A method according to the invention is used to calibrate a radio-based keyless entry system of a motor vehicle. A first transmitting device of the motor vehicle emits an LF field into the surroundings of the motor vehicle. The LF field is an electromagnetic field with a frequency from the LF band (LF: "low frequency"), i.e., a frequency in the range from 30 kHz to 300 kHz. This low-frequency LF field can advantageously be emitted with particularly low energy consumption and therefore permanently or continuously. Using a signal transmitter separate from the motor vehicle, in particular a portable one, a local field strength of the LF field is measured at the respective location of the signal transmitter. Based on this measured field strength of the LF field, it is then automatically determined whether the signal transmitter is located at a maximum of a specified distance from the motor vehicle. Predefined standard conditions can be used for this purpose.In other words, a characteristic map, a function, or the like can be specified and stored, which describes the decreasing field strength of the LF field with increasing distance from the motor vehicle under the specified standard conditions. This determination of the - assumed - distance of the signal transmitter from the motor vehicle, based on the measured field strength of the LF field, can be carried out using a data or signal processing device.

[0015] This data or signal processing device can be arranged in the signal transmitter itself and / or in or on the motor vehicle. In the former case, the signal transmitter can send or transmit a data or control signal to the motor vehicle in order to activate or trigger at least one function or functionality of the motor vehicle or of the keyless entry system. In the latter case, the signal transmitter can, for example, transmit a measured value of the field strength to the motor vehicle or the data or signal processing device arranged in the motor vehicle, which then determines from this whether the signal transmitter is at most at the specified distance from the motor vehicle. The corresponding function or functionality can therefore be triggered, activated or carried out if and / or as long as the signal transmitter is no further than the specified distance from the motor vehicle.Likewise, for example, the and / or another function or functionality can be activated or triggered if the thus determined distance of the signal transmitter from the motor vehicle exceeds the specified distance, i.e., the signal transmitter can no longer be detected within the specified distance from the motor vehicle. In the latter case, the motor vehicle can, for example, be automatically locked. In this way, the radio-based keyless entry system serves to provide at least one function or functionality at the specified distance from the motor vehicle.

[0016] The specified distance can depend on the respective function or functionality. For example, different distances can be specified for different functions or functionalities. For example, it can be provided that a lighting device of the motor vehicle is automatically activated when the signaling device has approached at least a first specified distance, for example, 6 m, from the motor vehicle. Once the signaling device has approached a second specified distance, for example, 2 m, from the motor vehicle, a second function can be triggered, for example, unlocking the motor vehicle.

[0017] According to the invention, if a predetermined condition coupled to the signal transmitter and / or to operation of the motor vehicle is met, a calibration signal with a higher frequency than the LF field is transmitted by means of a second transmitting device of the motor vehicle. The signal transmitter receives this calibration signal and then transmits a response signal with a frequency that is also higher than the LF field - in particular with the same frequency as the calibration signal - back to the motor vehicle. A reference distance of the signal transmitter from the motor vehicle is automatically determined from a signal propagation time of the calibration signal and the response signal between the transmission of the calibration signal and the reception of the response signal by the motor vehicle. The motor vehicle, for example the second transmitting device, can therefore in particular comprise a receiving device for the response signal.The reference distance represents in particular the actual distance of the signal generator from the motor vehicle, at least within the scope of the respective measurement accuracy.

[0018] The determination, based on the measured field strength of the LF field, of whether the signal transmitter is at most within the specified distance from the motor vehicle is then automatically calibrated depending on the reference distance determined based on the signal propagation time. In other words, the distance of the signal transmitter from the motor vehicle is determined in two different and independent ways, in particular using two different frequencies. The reference distance determined based on the signal propagation time (time of flight) serves as a reference or target value, which is also to be provided by the distance determination based on the measured LF field strength as a result for the corresponding actual position or distance of the signal transmitter relative to the motor vehicle. This is set, i.e. ensured, by calibration.The distance determination based on the signal propagation time is preferably carried out in particular when there is a direct line-of-sight connection between the motor vehicle, in particular the second transmitting device, and the signal transmitter, i.e. the calibration signal and the response signal can be transmitted directly between the motor vehicle and the signal transmitter, i.e. can be exchanged or transmitted without reflections.

[0019] In this way, an influence that the environment, in particular the ground, of the motor vehicle can have on the propagation and ultimately on the distance-dependent field strength or field strength change of the LF field can advantageously be automatically taken into account. For example, the measured field strength of the LF field, with the same transmission or output power of the first transmission device, at a certain distance from the motor vehicle can have a higher value if the motor vehicle is located in a reinforced concrete structure, for example a parking garage, than if the motor vehicle is located outdoors on a lawn, for example. This can lead to the assumed distance of the motor vehicle determined based on the measured LF field strength, i.e. the field strength of the LF field, being smaller than the reference distance and in particular than the actual or real distance of the signal transmitter from the motor vehicle.The latter can be determined with greater accuracy based on the signal propagation time of the calibration signal and the response signal. Accordingly, it can be provided, for example, that the corresponding function or functionality is only actually triggered when, after calibration or taking into account the specific reference distance, it has been determined or confirmed that the signal transmitter is actually located at most the specified distance from the motor vehicle. This can apply at least to future function triggering or determinations of the respective distance based on the respective measured field strength of the LF field, in particular as long as the motor vehicle is at its current position or in its current environment, i.e., has moved less than a specified distance since the last time the reference distance was determined.The reference distance can therefore be determined automatically, and the calibration signal can be automatically transmitted, whenever the vehicle has moved at least a specified distance. Likewise, other predefined conditions for the transmission of the calibration signal, i.e., for the re-determination of the reference distance, are possible, which are explained further below.

[0020] The fact that the condition is linked to the signaling device and / or to the operation of the motor vehicle means that it is not arbitrary, but rather is related to the signaling device, the motor vehicle, and / or their operation or behavior, i.e., bound to them. For example, the condition can be a specific switching state or process, a function activation or operating action of the signaling device and / or the motor vehicle performed automatically or manually by the user, and / or similar actions.The condition can be met, for example, if it has been checked with a positive result whether a speed of the motor vehicle has dropped below a predetermined threshold value, an engine or ignition of the motor vehicle has been or is switched off, a vehicle door of the motor vehicle has been opened, a button on the signal generator has been pressed and / or the like.

[0021] The present invention has the advantage that, due to automatic calibration, the accuracy of distance determination is improved even when the distance is determined based on the respectively measured field strength of the LF field. This can be carried out automatically and transparently for the user, i.e., unnoticed by the user. In particular, the access system or the motor vehicle can be flexibly adapted to different environments, i.e., always exhibit consistent behavior even in different environments, situations, or conditions. This is particularly advantageously made possible or achieved without excessively high energy consumption, as would occur, for example, with continuous use of the second transmitting device operated at a higher frequency and continuous or exclusive distance determination based on the signal propagation time.

[0022] The described method can be understood or referred to as calibration or calibration of the LF field using the time-of-flight measurement. This calibration advantageously eliminates the need to adjust the radio-based keyless entry system to a worst-case scenario or parameterize it for a corresponding worst-case scenario, as was previously the case, but can instead achieve its full potential depending on the situation. Overall, the present invention can therefore advantageously ensure that the respective user or customer perceives certain vehicle functions or function activations at the same distance from the vehicle, i.e., with the same range, regardless of external influences or conditions.

[0023] Thus, the disadvantage of an over-range of the LF field, which can be observed depending on the environment or the ground, due to the influence of different vehicle surfaces, dynamic parking scenarios and / or structural boundary conditions, can be avoided or compensated.

[0024] In an advantageous embodiment of the present invention, a frequency from the SHF band is used for the calibration signal and the response signal. In other words, a frequency from the range of 3 GHz to 30 GHz is used. This advantageously enables precise and reliable distance determination. Known radio technologies or standards, such as UWB (ultrawideband), Bluetooth, Wi-Fi, and the like, can preferably be used for this purpose. Available technology can therefore advantageously be used to determine the reference distance, which advantageously enables a particularly cost-effective and reliable implementation, as well as multiple use of the second transmitting device for other functions. Specifically, a frequency of 6.6 GHz, for example, can be used.

[0025] According to the invention, an LF reference signal with a predetermined transmission power is transmitted by the first transmitting device, and a field strength of the LF reference signal is measured by an LF receiving device arranged on the motor vehicle at a fixed distance from the first transmitting device. The LF receiving device thus provides a readback channel for the LF reference signal in or on the vehicle. A correction factor is automatically determined from the measured field strength of the LF reference signal, the predetermined transmission power, and the distance between the first transmitting device and the LF receiving device. This correction factor indicates an environmentally-dependent deviation of the measured field strength of the LF reference signal from a predetermined reference field strength. The predetermined reference field strength is, in particular, the field strength that would be measured for the LF reference signal using the LF receiving device under predetermined standard conditions.The reference field strength is therefore an expected field strength at the location of the LF receiving device. The correction factor is then also taken into account for calibrating the determination of whether the signal transmitter is located at or below the specified distance from the vehicle. In other words, the effect or influence of a current environment, in particular the current surface on which the vehicle is located, on the propagation or distance-dependent attenuation of the LF field is directly measured.

[0026] According to the invention, the LF reference signal is radiated or transmitted toward the background. The LF receiver then receives, and thus measures, the LF reference signal reflected from the background. This ensures that any influence of the background is actually taken into account in the measured field strength of the LF reference signal.

[0027] In the simplest case, the correction factor can be a number that quantifies, for example, a percentage deviation or attenuation of the measured field strength compared to the specified reference field strength. However, the correction factor can also be, for example, a characteristic map, a matrix, or a function. The correction factor can then, for example, specify different deviations or attenuation values ​​for different distances and / or directions. The latter can be particularly the case if the first transmitting device comprises multiple transmitting antennas and / or the LF receiving device comprises multiple receiving antennas. This then enables a direction-resolved measurement of the field strength. The LF reference signal can correspond to the LF field, i.e., be the LF field that is emitted anyway.However, the LF reference signal can also be a dedicated signal, which, in particular, may have the same frequency as the LF field, but is only transmitted in a different direction than the LF field at one or more specific predefined times and / or when a predefined condition—for example, the one already mentioned—is met. For this purpose, the first transmitting device can, for example, have a dedicated transmitting antenna for transmitting the LF reference signal. Additionally or alternatively, a transmitting antenna of the first transmitting device used for transmitting the LF field can be used for transmitting the LF reference signal.

[0028] Due to the defined, known, and particularly fixed, i.e., unchangeable, relative arrangement of the first transmitting device and the LF receiving device, a relationship between the field strength and the range can be established through the transmission and reception of the LF reference signal. This allows for conclusions to be drawn about environmental influences that can affect the field strength or range, and ultimately for responses to be made. Although the signal transmitter also has the means to measure the field strength of the LF reference signal, it is not always reliably located at a precisely known position or distance from the first transmitting device. Therefore, the field strength of the LF field measured by the signal transmitter cannot be used for calibration.

[0029] The first transmitter, the LF receiver, and the aforementioned data or signal processing device can form a closed control loop, by means of which the influence of the environment or the background on the LF field, or its propagation or range, can be controlled as a manipulated variable. This is advantageously possible with particularly low additional energy consumption, since the LF field or the first transmitter is used. Furthermore, the specified transmission power with which the LF reference signal is transmitted can be lower than the transmission power used to transmit the LF field, since, for example, only the environmental change and not necessarily the absolute field strength needs to be measured or taken into account.The signal processing device can, for example, serve as the actuator of the control loop by adapting an evaluation matrix or calculation rule or the like used to determine the distance depending on the measured field strength or the correction factor.

[0030] The first transmitting device and the LF receiving device can be arranged, for example, on opposite side sills of the motor vehicle. The first transmitting device and / or the LF receiving device can each be designed as a 1D antenna or 1D coil. This can be implemented with particularly low material, component, and cost requirements. Likewise, the first transmitting device and / or the LF receiving device can each be designed as 3D antennas or each comprise at least one 3D antenna, which advantageously enables more flexible placement or arrangement in or on the motor vehicle without loss of performance.

[0031] In a further advantageous embodiment of the present invention, a predetermined threshold value of the field strength of the LF field, associated with the predetermined distance, is automatically and dynamically adjusted for or as the calibration, i.e., during the calibration. To determine whether the signal transmitter is located at or below the predetermined distance from the motor vehicle, the measured field strength of the LF field is then compared with the adjusted threshold value. This determination of the distance of the signal transmitter, based on the measured field strength of the LF field, by comparing the respectively measured field strength with a predetermined threshold value that corresponds to or is associated with the predetermined distance, can be applied to any embodiment of the method according to the invention.Dynamically adjusting the threshold value for calibration means that the assignment or the specified dependency between the distance and the measured field strength is adjusted, i.e. changed if necessary.

[0032] For example, the signaling device can be positioned at exactly the specified distance, either originally or by the manufacturer, and the field strength of the LF field can be measured at this position or at this distance under specified standard conditions. Accordingly, a measured field strength value is then assigned to the specified distance. This can, for example, be carried out once by the manufacturer before delivery of the access system or the motor vehicle. If the motor vehicle is then used in a later application, for example, in an environment that favors the propagation of the LF field compared to the specified standard conditions, a higher or greater field strength can be measured if and even if the signaling device is actually also located at the specified distance.In such a case, a higher threshold value would be assigned to the specified distance, which, prior to this adjustment, would have been interpreted as indicating a shorter distance compared to the specified distance. In this way, the access system can be dynamically adjusted, i.e., calibrated, particularly easily and repeatedly. Since only a data-based assignment or evaluation of the respective measured values ​​is adjusted here, it is advantageously possible to dispense with, for example, a technically more complex control or regulation or adjustment of the first transmitting device and / or the transmitting power used by it to emit the LF field.

[0033] In a further advantageous embodiment of the present invention, a transmission power of the first transmission device used to emit the LF field is automatically and dynamically adjusted for or as the calibration, i.e. during the calibration. In the example described above, in which the motor vehicle is located in the environment that supports the propagation of the LF field, the transmission power of the first transmission device or of the LF field could then be reduced. This can then ensure that the field strength of the LF field measured or expected under standard conditions is present, i.e., measured, at the specified distance in the new environment as well. This advantageously makes it possible to realize the desired consistent behavior of the access system with at least an average reduction in energy requirement or energy consumption.

[0034] A combination of the aforementioned calibration measures is also possible. This can be particularly useful if the range of possible adjustment of the transmission power is insufficient to compensate for the respective environmental influences.

[0035] In a further advantageous embodiment of the present invention, the predetermined condition checked is whether the signal transmitter is moving away from the motor vehicle or approaching the motor vehicle. In other words, the calibration signal can be emitted when it is determined or detected that the signal transmitter is moving away from the motor vehicle and / or when it is determined or detected that the signal transmitter is approaching the motor vehicle. In such a case, it can be assumed that the motor vehicle is not in active traffic, i.e. the respective environmental conditions are not expected to change immediately. It can therefore then be assumed that the functionality of the access system is likely to be used before the motor vehicle is moved to another environment."By calibrating the access system as described in such a situation, the expected, consistent distance-dependent behavior can be set or ensured, at least for the next functional use. In this case, a direction from which the signaling device is approaching or in which it is moving away can also be detected and taken into account. With a corresponding direction-resolving or direction-sensitive design of the access system, in particular of the first transmitter device, calibration can then be carried out direction-dependently, i.e., for the respective detected direction of the signaling device.

[0036] In a further advantageous embodiment of the present invention, the occurrence of a field strength jump in the field strength of the LF field is used or evaluated as the predetermined condition. In other words, the LF field strength is measured continuously or regularly - by means of the signal transmitter and / or by means of the LF receiver device - and a sudden or discontinuous change in the field strength is detected. Such a field strength jump can occur, for example, at a transition between two different surfaces or types of surfaces, i.e., in two different and non-continuously merging environments or environmental conditions. In particular, such a field strength jump can occur when or while the motor vehicle is moving. This can be the case, for example, during a maneuver, particularly an automatic or remote-controlled maneuver, when entering or leaving a parking space.In such a case, the signal transmitter can be located outside the motor vehicle despite its movement, for example, so that the reference distance can be determined particularly accurately and reliably based on the signal propagation time. Because the specified condition is considered to be met when such a jump in field strength, for example of a specified strength or intensity, occurs, i.e. is detected or recorded, it is advantageously always possible to react to a new or changed environment that differs from the previous environment at least to a specified extent with regard to an attenuation property or a propagation characteristic of the LF field. This ensures that the access system is automatically calibrated in every environment without the need to permanently or continuously transmit the calibration signal and determine its signal propagation time.Thus, a particularly good compromise is advantageously achieved between the energy requirement for the access system, in particular the calibration, and the accuracy of the access system or the distance determination.

[0037] In a further advantageous embodiment of the present invention, a time grid is used or evaluated as the predetermined condition, according to which the calibration signal is transmitted after a predetermined number of determinations of the distance of the signal transmitter from the motor vehicle based on the measured field strength of the LF field. In other words, the predetermined condition checks how often the distance of the signal transmitter from the motor vehicle has been determined by measuring the field strength of the LF field since the last reference distance was determined based on the signal propagation time of the calibration signal and the corresponding response signal. If this number corresponds at least to the predetermined number, the calibration signal is automatically transmitted and the reference distance is determined.For example, at or after every twentieth distance determination based on the measured field strength of the LF field, the reference distance can be determined based on the signal propagation time, and the access system can be calibrated. This condition can be checked and monitored particularly easily and without additional measurement effort. This also advantageously achieves a particularly good compromise between the energy consumption and the accuracy of the access system.

[0038] A further aspect of the present invention is a radio-based keyless access system for a motor vehicle, comprising a portable signal transmitter, a first transmitting device for the motor vehicle for emitting an LF field, a second transmitting device for the motor vehicle for emitting a calibration signal with a higher frequency than the LF field, and a data or signal processing device. According to the invention, the access system, in particular the signal processing device, is configured to automatically carry out at least one variant of the method according to the invention. The access system according to the invention can, in particular, be the access system mentioned in connection with the method according to the invention.The access system, in particular the signal processing device, can comprise a data memory in which a program code is stored that encodes or represents the method steps of the method according to the invention. Furthermore, the access system, in particular the signal processing device, can comprise a processor device coupled to this data memory for executing the program code. Furthermore, the access system according to the invention can comprise further parts or components, such as associated cabling, connectors, connections, housings, interfaces, and / or the like.

[0039] The signal generator, especially a portable one, can be a separate device or integrated into a vehicle key for the motor vehicle. Additionally or alternatively, a mobile device such as a smartphone, a smartwatch, a wearable device, or the like can also be used as the signal generator.

[0040] A further aspect of the present invention is a motor vehicle with a radio-based keyless entry system according to the invention. The motor vehicle according to the invention can, in particular, be the motor vehicle mentioned in connection with the access system according to the invention and / or in connection with the method according to the invention. Accordingly, the motor vehicle according to the invention can also have the properties and / or parts or components described therein. This can, for example, relate to the LF receiving device and / or a receiving device for the response signal.

[0041] In a further preferred embodiment of the invention, the access system according to the invention or the motor vehicle according to the invention can have an environment detection device. This can, for example, comprise at least one camera for detecting the environment of the motor vehicle, in particular the ground in the area or surroundings of the motor vehicle. An image processing or image recognition system or algorithm can then be provided which, based on image data of the environment or ground supplied by the camera, categorizes or classifies the respective environment or ground, i.e., determines or classifies it with regard to its type.Furthermore, an assignment table, a characteristic map, or the like can be specified, which indicates a relationship or assignment between different environments, substrates, or substrate types and a corresponding effect on the LF field, in particular a corresponding attenuation or propagation characteristic of the LF field, in these environments and / or on or above these substrates or substrate types. The respectively detected environment or the respectively detected substrate can then also be used or taken into account for calibration together with the specified assignment table or the specified characteristic map.

[0042] Likewise, the plausibility of the distance determination based on the measured field strength of the LF field and / or the signal propagation time and / or the determination of the correction factor can be checked based on the detected environment or the detected background and the specified assignment table or the specified characteristic map. By appropriately comparing the respective measured or determined values ​​with the type of background or environment, it can be concluded, for example, that a local peculiarity exists in the event of significant deviations or discrepancies. For example, the vehicle may be parked in a meadow but happen to be located directly above a metallic object. In such a case, a discrepancy may arise between the measured attenuation and the attenuation expected based on the image data of the background.It can then be assumed, for example, that the calibration carried out locally at this particular location may be unreliable, which is why, for example, predefined standard values ​​can be used to determine the distance.

[0043] The invention also includes further developments of the access system according to the invention and the motor vehicle according to the invention, which have features as described in connection with the further developments of the method according to the invention, and vice versa. To avoid unnecessary redundancy, the corresponding further developments of the invention are not described separately here for each aspect of the present invention.

[0044] An exemplary embodiment of the invention is described below. The single figure shows a schematic overview illustrating a method for calibrating a radio-based keyless entry system of a motor vehicle.

[0045] The exemplary embodiment explained below is a preferred embodiment of the invention. In the exemplary embodiment, the described components of the embodiment each represent individual, independently considered features of the invention, which also further develop the invention independently of one another and are thus also to be considered as components of the invention, either individually or in a combination other than that shown. Furthermore, the described embodiment can also be supplemented by further features of the invention already described.

[0046] The single figure shows a schematic overview to illustrate a method for calibrating a radio-based keyless entry system of a motor vehicle 1. The motor vehicle 1 has a first transmitting device 2 for emitting an LF field 3, indicated schematically here, into the surroundings of the motor vehicle 1. The motor vehicle 1 also has an LF receiving device 4 for receiving or measuring signals or fields emitted by the first transmitting device 2, in particular their field strength. The motor vehicle 1 also has a second transmitting device 5 for emitting signals or fields at a higher frequency than the LF field 3. In this case, the second transmitting device 5 comprises a receiving device for signals or fields of this higher frequency. Finally, the motor vehicle 1 has a signal processing device 6 and an environment detection device 14.The signal processing device 6 is connected by corresponding data connections to the first transmitting device 2, the LF receiving device 4, the second transmitting device 5 and the environment detection device 14, for example via an on-board network of the motor vehicle 1.

[0047] In the surroundings of the motor vehicle 1, a signal transmitter 7 is shown here. This signal transmitter 7 is part of the radio-based keyless entry system for the motor vehicle 1. By means of this access system, at least one predetermined function or functionality of the entry system or of the motor vehicle 1 is to be automatically triggered as soon as the signal transmitter 7, for example worn by a user of the motor vehicle 1, has approached the motor vehicle 1 to within a predetermined distance. For this purpose, the current distance of the signal transmitter 7 from the motor vehicle 1 must be determined, since the respective function or functionality is to be provided or triggered only up to the predetermined distance, i.e. up to a certain range limit, and not at greater distances.

[0048] In this case, the signal transmitter 7 measures the field strength of the LF field 3 currently present at its location and transmits a corresponding data signal 8 to the motor vehicle 1, where it is further processed or evaluated by the signal processing device 6. From the data signal 8 and a predetermined threshold value, which indicates a field strength of the LF field 3 associated with the predetermined distance, an assumed distance - here corresponding to a position 9 - of the signal transmitter 7 is then determined. Based on the field strength of the LF field 3 measured by the signal transmitter 7, it is automatically determined whether the signal transmitter 7 is at most the predetermined distance from the motor vehicle 1. If this is the case, the respective function would then be triggered accordingly.However, depending on the surface surrounding the motor vehicle 1, the propagation characteristics or propagation behavior of the LF field 3 may vary unexpectedly. This may result in the assumed distance or position 9 of the signal generator 7 deviating from its actual distance—here, corresponding to an actual position 10.

[0049] To counteract this problem, it is provided that the access system, i.e. the automatic determination of the distance of the signal transmitter 7 from the motor vehicle 1, is automatically calibrated based on the measured field strength of the LF field 3. For this purpose, a calibration signal 11 is transmitted by the second transmitting device 5 when a predetermined condition coupled to the signal transmitter 7 and / or to the operation of the motor vehicle 1 is met. In response to the calibration signal 11, a corresponding response signal 12 is transmitted from the signal transmitter 7 to the motor vehicle. The signal processing device 6 then automatically determines a reference distance of the signal transmitter 7 from the motor vehicle 1 based on the signal propagation time between the transmission of the calibration signal 11 and the reception of the response signal 12 in the motor vehicle 1.This distance determination by means of a second frequency is particularly safe and reliable, since a frequency from the SHF band, for example 6.6 GHz, is used for the calibration signal 11 and the response signal 12, while the LF field 3 has a frequency between 100 kHz and 150 kHz, for example.

[0050] The determination, based on the measured field strength of the LF field 3, as to whether the signal transmitter 7 is located at a maximum of the specified distance from the motor vehicle 1 is then automatically calibrated by the signal processing device 6 as a function of the reference distance determined based on the signal propagation time of the calibration signal 11 and the response signal 12. Thus, the LF field 3 is measured or the distance determination is calibrated using a combination of different signals or fields, i.e., by exploiting different properties of different field types or field types. This allows the influence of the environment of the motor vehicle 1 on the distance-dependent field strength of the LF field 3 and thus on the distance determination of the signal transmitter 7 based thereon to be taken into account.This avoids the need to restrict the access system more than necessary in all operating states other than the worst case scenario. Overall, this allows an improvement in the range limit setting of the access system for situation-specific influences without placing excessive strain on the energy balance of the motor vehicle 1 and / or the signal transmitter 7.

[0051] For further improvement, it is provided here that an LF reference signal 13 with a predetermined transmission power is transmitted by the first transmitting device 2 and received by the LF receiving device. The LF receiving device is arranged at a fixed distance from the first transmitting device 2 on the motor vehicle 1. A field strength of the LF reference signal 13 measured at the location of the LF receiving device 4 is determined by the LF receiving device 4 or the signal processing device 6. From this, a correction factor is determined by the signal processing device 6, which indicates an environmentally-related deviation of the measured field strength of the LF reference signal 13 from a predetermined reference field strength. This correction factor is also taken into account by the signal processing device 6 for calibration.

[0052] The environment detection device 14 can, for example, comprise two cameras for detecting the ground in the surroundings of the motor vehicle 1. Image data acquired by the environment detection device 14 are automatically evaluated by the signal processing device 6 to determine a type of ground in the surroundings of the motor vehicle 1. The signal processing device 6 then uses the determined type of ground—for example, concrete, asphalt, grass, earth, or the like—and a corresponding mapping table between different ground types and associated propagation characteristics of the LF field 3 to verify the plausibility of the distance determination and / or calibration.

[0053] Overall, the described examples show how the accuracy of a radio-based keyless entry system of a motor vehicle 1 can be improved. List of reference symbols

[0054] 1Motor vehicle 2First transmitting device 3LF field 4LF receiving device 5Second transmitting device 6Signal processing device 7Signal generator 8Data signal 9Assumed position (of signal generator 7) 10Actual position (of signal generator 7) 11Calibration signal 12Response signal 13LF reference signal 14Environment detection device

Claims

1. Method for calibrating a radio-based keyless access system of a motor vehicle (1), in which: - by means of a first transmitting device (2) of the motor vehicle (1), an LF field (3) is emitted into an environment of the motor vehicle (1), and - based on a field strength of the LF field (3) measured by means of a signal generator (7) separate from the motor vehicle (1), it is automatically ascertained whether the signal generator (7) is located no further than a predetermined distance from the motor vehicle (1), wherein - if a predetermined condition coupled to the signal generator (7) and / or to an operation of the motor vehicle (1) is fulfilled, a calibration signal (11) having a higher frequency than the LF field (3) is transmitted by means of a second transmitting device (5) of the motor vehicle (1), and in response, the signal generator (7) then sends back to the motor vehicle (1) a response signal (12) having a frequency that is also higher than the LF field (3), - a reference distance (10) of the signal generator (7) from the motor vehicle (1) is automatically ascertained from a signal delay between transmitting the calibration signal (13) and receiving the response signal (12) by the motor vehicle (1), and - the ascertainment, based on the measured field strength of the LF field (3), of whether the signal generator (7) is located no further than the predetermined distance from the motor vehicle (1) is automatically calibrated depending on the reference distance (10) ascertained on the basis of the signal delay; wherein an LF reference signal (13) having a predetermined transmitting power is transmitted by means of the first transmitting device (2), and a field strength of the LF reference signal (13) is measured by means of an LF receiving device (4) arranged on the motor vehicle (1) at a fixed distance from the first transmitting device (2), - a correction factor is ascertained from the measured field strength of the LF reference signal (13), the predetermined transmitting power and the distance between the first transmitting device (2) and the LF receiving device (4), which correction factor indicates an environmental deviation of the measured field strength of the LF reference signal (13) from a predetermined reference field strength, and - the correction factor is also taken into account for calibrating the ascertainment of whether the signal generator (7) is located no further than the predetermined distance from the motor vehicle (1), wherein the LF reference signal (13) is transmitted in the direction of the ground and an LF reference signal reflected from the ground is received by means of the LF receiving device (4), and an influence of the ground is taken into account in the measured field strength of the LF reference signal (13).

2. Method according to claim 1, characterized in that a frequency from the SHF band is used for the calibration signal (11) and the response signal (12).

3. Method according to any of the preceding claims, characterized in that - for the calibrating, a threshold value of the field strength of the LF field (3), which threshold value is assigned to the specified distance, is automatically dynamically adjusted, and - for the ascertainment of whether the signal generator (7) is located no further than the predetermined distance from the motor vehicle (1), the measured field strength of the LF field (3) is compared with the adjusted threshold value.

4. Method according to any of the preceding claims, characterized in that - for the calibrating, a transmitting power of the first transmitting device (2), which transmitting power is used to emit the LF field (3), is automatically dynamically adjusted.

5. Method according to any of the preceding claims, characterized in that whether the signal generator (7) is moving away from the motor vehicle (1) or approaching the motor vehicle (1) is checked as the predetermined condition.

6. Method according to any of the preceding claims, characterized in that the occurrence of a field strength jump in the field strength of the LF field (3) is used as the predetermined condition.

7. Method according to any of the preceding claims, characterized in that a timeframe is used as the predetermined condition, according to which timeframe the calibration signal (11) is transmitted after the distance of the signal generator (7) from the motor vehicle (1) has been ascertained a predetermined number of times based on the measured field strength of the LF field (3).

8. Radio-based keyless access system for a motor vehicle (1), comprising a portable signal generator (7), a first transmitting device (2) for the motor vehicle (1), for emitting an LF field (3) and an LF reference signal (13), an LF receiving device (4) for the motor vehicle, a second transmitting device (5) for the motor vehicle (1), for emitting a calibration signal (11) having a higher frequency than the LF field (3), and a signal processing device (6), characterized in that the access system is configured to automatically carry out a method according to any of the preceding claims.

9. Motor vehicle (1) comprising a radio-based keyless access system according to claim 8.