Electronic key and method for the energy-saving operation of such a key

Abstract

The invention relates to an electronic key (1) for authentication with respect to a connection partner (2) and for determining the distance to the connection partner (2), wherein a control module (10) is designed to compare a sensor signal (20) from a motion sensor (13) with a motion limit value (21) and to switch the first radio module (11) from its inactive state into its active state and to control it to set up the radio connection (30) if the sensor signal (20) is greater than the motion limit value (21), and is designed to compare the signal strength of the radio connection (30) with a signal limit value (31), to control the first radio module (11) to send a request to determine the distance with low priority (H) to the connection partner (2) via the radio connection (30) if the signal strength is less than the signal limit value (31), and to check the sensor signal (20) for a predetermined signal pattern (23) if the signal strength is greater than the signal limit value (31), wherein the control module (10) is additionally designed to control the first radio module (11) to send a request to determine the distance with medium priority (K) to the connection partner (2) via the radio connection (30) if the predetermined signal pattern (23) is not registered within a predetermined first time period, and to control the first radio module (11) to send a request to determine the distance with high priority (L) to the connection partner (2) via the radio connection (30) if the predetermined signal pattern (23) is registered within the first predetermined time period.

Description

[0001] Marquardt GmbH

[0002] P 646 PCT WE / Kn

[0003] Electronic key and method for energy-saving operation of such a key

[0004] Description:

[0005] The invention relates to an electronic key for authentication with a communication partner and determination of the distance to the communication partner, as well as a method for energy-saving operation of such an electronic key, wherein the communication partner is in particular a vehicle.

[0006] For the purposes of the present invention, electronic keys are understood to be all electronic devices, particularly battery-operated ones, such as radio keys or key fobs, which are designed and whose primary purpose is authentication vis-à-vis the communication partner and determining the distance between the electronic key and the communication partner. Mobile phones and smartphones are specifically excluded from the definition of electronic keys, as although they can serve as such, they are not specifically designed for this purpose.

[0007] In principle, a large number of different electronic keys are known from the state of the art, which communicate with the connection partner via a radio link and serve for authentication vis-à-vis the connection partner and often also for determining the distance between electronic keys and connection partners.

[0008] These are powered by a battery or accumulator, so each authentication and distance measurement reduces the lifespan of this energy source.

[0009] With classic electronic keys that operate in the LF and / or HF range, authentication and, if necessary, distance determination are performed relatively rarely and, for example, only after activation by a user, so that the resulting load or reduction in the lifespan of the energy source is acceptable.

[0010] However, with electronic keys where authentication and distance determination are based on BLE (Bluetooth Low Energy) and UWB (Ulta-Wideband), attempts are repeatedly made at short intervals to establish a radio connection between the key and the other device. If a connection is successfully established, meaning the user with the electronic key is in close proximity to the other device, distance measurements are repeatedly taken at short intervals. This constant attempt to establish a radio connection and the distance measurement drastically reduce the battery life, necessitating charging or replacement of the battery.

[0011] Given that BLE enables a radio connection up to, for example, 180 m distance, this is relatively common.

[0012] Accordingly, the battery life of such BLE and UWB-based keys is particularly affected by the usage profile, i.e., how frequently the user is in close proximity to the connection partner, thus reducing user comfort.

[0013] The invention is therefore based on the objective of overcoming the aforementioned disadvantages and providing an electronic key and a method which enable energy-saving operation of the electronic key and extend the battery life accordingly.

[0014] This task is solved by the combination of features according to the independent claims.

[0015] According to the invention, an electronic key is proposed for authentication with a communication partner and for determining the distance to that communication partner, wherein the communication partner is preferably a vehicle or a vehicle control unit designed for this purpose. The key comprises a control module, a first radio module for establishing a radio connection with the communication partner to enable information exchange, a second radio module for determining the distance to the communication partner, and a motion sensor configured to generate a sensor signal dependent on movement of the electronic key. As explained below, the control module can be provided as a separate circuit or integrated into one of the radio modules or into the motion sensor.The first radio module is preferably a BLE radio module, and the second radio module is preferably a UWB radio module. The fact that the second radio module serves to determine, or is suitable for determining, the distance to the communication partner is to be understood as meaning that the second radio module participates in determining the distance to the communication partner, but does not necessarily perform this determination actively. For example, it is sufficient if the second radio module receives radio signals from the communication partner and then transmits radio signals back and / or extracts information from the radio signals of the communication partner and processes it. In general, such distance determination is referred to as ranging in the context of the invention.To enable control and signal exchange, the control module is preferably connected to the first radio module, the second radio module, and the motion sensor via signal transmission. At least the first radio module is also switchable between an active and an inactive state, with its energy consumption being reduced in the inactive state compared to the active state. In the inactive state, the first radio module can also be considered to be in standby mode or switched off. Furthermore, the first radio module is configured to determine the signal strength of the radio connection, which is preferably expressed as an RSSI value (Received Signal Strength Indicator).

[0016] The essential aspect of the invention is that the control module is configured to compare the sensor signal with a motion threshold and to switch the first radio module from its inactive state to its active state and initiate the radio connection if the sensor signal exceeds the motion threshold. Furthermore, the control module is configured to compare the signal strength with a signal threshold and to send the first radio module a low-priority distance determination request to the communication partner via the radio link if the signal strength is below the signal threshold. Finally, the control module checks the sensor signal for a predetermined signal pattern if the signal strength exceeds the signal threshold. If the signal pattern is not registered within a predetermined initial time period, i.e.,If the signal pattern is detected, the first radio module is activated by the control module and sends a distance determination request with medium priority to the communication partner via the radio link. If the signal pattern is registered within the predetermined initial time period, the first radio module is activated by the control module and sends a distance determination request with high priority to the communication partner via the radio link.

[0017] The low, medium, and high priority requests should preferably be understood as corresponding to a desired access or a probability of access or approach. If the request signals a low priority, the probability that the user will approach the connection partner is low. If the request signals a medium priority, the probability is moderate, and if the request signals a high priority, the probability is high. Actions can then be taken accordingly. For example, it could be stipulated that ranging, i.e., determining the distance, is only performed for a high-priority distance request. Furthermore, it could be stipulated that ranging is performed in the case of repeated medium-priority distance requests.The same applies to low-priority distance queries, where the number required to trigger a ranging is, for example, twice the number required for medium-priority distance queries.

[0018] The motion threshold should preferably be chosen such that it can be assumed that the user's movement is intentional and not accidental. For example, the motion threshold can be chosen to correspond to the value, signal, or curve that the motion sensor detects when the user takes a step. Alternatively, and as mentioned later, the motion threshold can be an acceleration value, chosen analogously so that it is exceeded, for example, when the user takes several steps or begins a sequence of steps.

[0019] Regarding the signal threshold, it is important to note that this correlates with a distance to the connection partner at which the user's intention to access the connection partner, thus necessitating authentication and distance determination, is likely. For example, an RSSI value corresponding to a distance of 3 m to the connection partner could be chosen as the signal threshold. This means that the signal strength exceeds the threshold when the user with the key(s) is within 3 m of the connection partner. A usable RSSI value for a BLE connection at a distance of 3 m would be, for example, -70 dBm.

[0020] Preferably, the signal pattern is generated by the motion sensor during a sequence of several, preferably at least three, consecutive steps taken by a user carrying the key, either immediately or within the first time interval. The sensor signal is then monitored and evaluated for this signal pattern. To avoid misidentifying widely spaced steps as such a signal pattern, it is required that these steps be detected within the first time interval. This allows for a high priority in distance determination, at which point the evaluation is aborted. Conversely, if no such signal pattern is detected within the first time interval, only a medium priority for distance determination is assumed.

[0021] For example, two seconds is chosen as the first time interval, although other time intervals in the range between 1 and 5 seconds would also be possible.

[0022] The motion limit, the signal limit, the signal pattern, the first time interval, and the time intervals mentioned below are stored in the control module, preferably in a non-volatile memory of the control module, and are particularly preferably changeable by means of a suitable interface.

[0023] Within the scope of the invention, motion sensors are preferably understood to be only those sensors which allow a direct inference about the movement of the key based on the measurement data. Thus, a GPS receiver or GPS sensor is not considered a motion sensor within the meaning of the invention, since it only allows an inference about movement by comparing current measurements with past measurements.

[0024] In an advantageous embodiment of the invention, the motion sensor is, for example, an acceleration sensor which detects the acceleration of the key. Consequently, and as already explained, the motion limit can be an acceleration limit. The control module can be provided or designed as a separate circuit. Alternatively, however, the control module can be integrated into the first radio module, the second radio module, or the motion sensor.

[0025] Regardless of whether the control module is designed as a separate circuit or integrated into one of the other modules, it is preferably intended that the control module utilizes existing components to further reduce energy consumption. For example, the electronic key can include a clock and, in particular, a 32 kHz low-power timer, which is required for a BLE radio link anyway and has low energy consumption, so it can also be used by the control module. The clock can then be used, for instance, to monitor the elapsed time intervals or to enable comparisons. It follows that the clock is integrated into the first radio module for establishing and maintaining the radio link, or is connected to the first radio module via a signal path and simultaneously to the control module via a signal path.

[0026] As already explained, the signal pattern is preferably a signal generated by the motion sensor during a sequence of several, for example three or at least three, consecutive steps by a user carrying the key.

[0027] To enable further energy savings and generally energy-efficient operation of the key, the control module can also be switched between an active and an inactive state. The energy consumption of the control module is also reduced in its inactive state compared to its active state. In the inactive state, the control module can also be considered to be in standby mode or switched off. Furthermore, the control module is configured to switch from the inactive state to the active state upon receiving a sensor signal and preferably remain in this state for a predetermined time before switching back to the inactive state without an incoming sensor signal.

[0028] Similarly, the second radio module can be switched between an active and an inactive state according to an advantageous embodiment. In its inactive state, the energy consumption of the second radio module is reduced compared to its active state, and the second radio module can also be considered to be in standby mode or switched off in its inactive state. In this embodiment, the control module is configured to switch the second radio module preferably directly from its inactive state to its active state when, or as soon as, the first radio module transmits a high-priority distance determination request to the communication partner, or when the first radio module receives a distance determination request from the communication partner.

[0029] To reduce energy consumption, the control module can also be configured to switch the first radio module and / or the second radio module and / or itself from its respective active state to its respective inactive state if the motion sensor does not detect any movement, or any movement above the motion threshold or a second motion threshold, within a predetermined second time period, and / or if the first radio module does not establish a radio connection with the communication partner within a predetermined third time period. Regarding all comparisons, a borderline case can be assigned to both cases where the comparison value is less than the threshold and cases where the comparison value is greater than the threshold.

[0030] A further aspect of the invention relates to a method for the energy-saving operation of an electronic key for authentication with a communication partner and for determining the distance to the communication partner, wherein this is preferably an electronic key proposed according to the invention. The key again comprises a control module, a first radio module for establishing a radio connection with the communication partner enabling information exchange, a second radio module for determining the distance to the communication partner, and a motion sensor configured to generate a sensor signal dependent on movement of the electronic key. Furthermore, the first radio module is preferably a BLE radio module and the second radio module is preferably a UWB radio module.The first radio module is switchable between an active state and an inactive state with reduced energy consumption and is designed to determine the signal strength of the radio connection.

[0031] The proposed method comprises at least the following steps, preferably in the given order: a. Comparison, preferably by the control module, of the sensor signal with a motion threshold, wherein the first radio module switches from its inactive state to its active state and establishes the radio link when or as soon as the sensor signal exceeds the motion threshold; b. Comparison, preferably by the control module, of the signal strength with a signal threshold, wherein the first radio module sends a low-priority distance determination request to the communication partner via the radio link when or as soon as the signal strength is less than the signal threshold, and checking, monitoring, or evaluating, preferably by the control module, the sensor signal for a predetermined signal pattern when or as soon as the signal strength exceeds the signal threshold; c.Controlling the first radio module, preferably by the control module, to send a distance determination request with medium priority to the communication partner via the radio link if or as soon as the predetermined signal pattern is not registered within a predetermined first time period, and controlling the first radio module, preferably by the control module, to send a distance determination request with high priority to the communication partner via the radio link if or as soon as the predetermined signal pattern is registered within the predetermined first time period.

[0032] Preferably, the second radio module can be switched between an active state and an inactive state with reduced energy consumption, and the method further comprises step d. switching, preferably by the control module, the second radio module from its inactive state to its active state when, or as soon as, the first radio module transmits a high-priority distance determination request to the communication partner, or when the first radio module receives a distance determination request from the communication partner. It should be emphasized that step d. preferably follows step c.

[0033] Furthermore, the control module can be switched between an active state and an inactive state with reduced energy consumption, and the method can also include the step: e. Switching the control module from the inactive state to the active state upon receiving the sensor signal.

[0034] Regarding step e., it should be noted that this can be carried out before step a. and serves to save further energy.

[0035] Furthermore, the method can include the step: f. Switching the first radio module and / or the second radio module and / or the control module, preferably by the control module, from the respective active state to the respective inactive state if the motion sensor does not detect any movement within a predetermined second time period, for example 5 seconds, and / or if the first radio module does not establish a radio connection with the communication partner within a predetermined third time period, which may be equal to the second time period.

[0036] The features disclosed above can be combined in any way, provided that this is technically possible and they do not contradict each other.

[0037] Other advantageous embodiments of the invention are characterized in the dependent claims or are described in more detail below together with the description of the preferred embodiment of the invention with reference to the figures. The figures show:

[0038] Fig. 1 shows a system comprising an electronic key; Fig. 2 shows a process flow based on a flowchart.

[0039] The figures are schematic examples. Identical reference symbols in the figures indicate identical functional and / or structural features.

[0040] Figure 1 schematically depicts an electronic key 1 and a communication partner 2, and thus a system comprising the key 1 and the communication partner 2. For example, the communication partner 2 is a motor vehicle or a control unit of a motor vehicle.

[0041] As is known in the prior art, the electronic key 1 serves to authenticate the user 3 carrying the electronic key 1 to the vehicle and to determine the distance of the electronic key 1 to the communication partner 2, in order to be able to trigger comfort functions such as the automatic unlocking of the vehicle doors on this basis.

[0042] The electronic key 1 includes a first radio module 11, which is designed as a BLE radio module, and a second radio module 12, which is designed as a UWB radio module, for authentication and distance determination.

[0043] Since a continuous radio connection 30 or BLE radio connection 30 of the first radio module 11 and the frequent ranging that occurs during the radio connection 30 (i.e., distance determination by the connection partner 2 and the second radio module 12) drastically reduces the lifespan of the energy source supplying power to the electronic key 1, the electronic key 1 is provided with a control module 10 and a motion sensor 13 configured as an accelerometer. The control module 10 first compares the sensor signal 20 generated by the motion sensor 13 with a movement threshold 21, specifically an acceleration threshold, which is selected such that minor movements are below the movement threshold 21 and the accelerations resulting from an action or step by the operator 3 are above the movement threshold 21.

[0044] If a sensor signal 20 detected by the motion sensor 21 exceeds the motion threshold 21, the first radio module 11 is switched from its inactive state to its active state and controlled in such a way that the radio connection 30 is established between the first radio module 11 and the connection partner 2. If it is not possible to establish the radio connection 30 because, for example, the connection partner 2 is not within range, the first radio module 11 is switched off again or back to the inactive state after a third time interval.

[0045] Once a radio connection 30 has been established, the signal strength determined by the first radio module 11 is compared by the control module 10 with a signal threshold 31, which corresponds to a predetermined distance X to the connection partner, beyond which an access or authentication request from the user to the connection partner 2 is likely. The distance X to the connection partner could, for example, be 3 m. If the signal strength is less than the signal threshold 31, the access request is correspondingly unlikely, so the first radio module 11 is activated by the control module 10 to send a low-priority request to the connection partner 2 via the radio connection 30 to determine the distance, according to step H in Figure 2. However, if the signal strength is greater than the signal threshold 31, the sensor signal 20 is monitored by the control module 10 for a predetermined signal pattern 23.checked, wherein the signal pattern 23 is a signal generated by the motion sensor 13 during a sequence of several, for example three, steps taken immediately or within a first time interval by a user 3 carrying the key 1.

[0046] If the signal pattern 23 is not registered within the predetermined first time period, the first radio module 11 is controlled by the control module 10 to send a request to determine the distance with medium priority to the communication partner 2 via the radio link 30 according to step L shown in Figure 2.

[0047] However, if the signal pattern 23 is registered within the predetermined first time period, the first radio module 11 is controlled by the control module 10 to send a high-priority distance determination request to the communication partner 2 via the radio link 30 in accordance with step M shown in Figure 2.

[0048] When key 1 is activated by sufficient movement, it transmits a distance-determining (ranging) request with low, medium, or high priority to the connection partner 2 via the first radio module 1. Based on this request, the connection partner 2 can decide, for example, depending on a stored setting, whether to perform a ranging operation.

[0049] Preferably, however, the second radio module 12 required for ranging is only activated during a high-priority distance determination request, or a first predetermined number of medium-priority distance determination requests, or a second predetermined number of low-priority distance determination requests, and thus much less frequently than would be the case conventionally, so that ranging and the associated energy consumption are significantly reduced.

[0050] Based on the system shown in Figure 1 or the electronic key 1 shown in Figure 1, the method shown in Figure 2 is implemented by the key 1 and in particular by the control module 10 of the key 1, which is based on a flowchart in its representation and goes through the following steps:

[0051] A Waiting for a sensor signal 20 generated by the motion sensor 13 or receiving a sensor signal 20 generated by the motion sensor 13.

[0052] B. Comparison of the sensor signal 20 received by the motion sensor 13 with the motion limit 21; if the sensor signal 20 is less than or equal to the motion limit 21, proceed to step C and if the sensor signal 20 is greater than the motion limit 21, proceed to step D.

[0053] C Switching the key 1 or the first radio module 11 and, if applicable, the control module 10 and / or the second radio module 12 and / or the motion sensor 13 into the respective inactive state.

[0054] D Controlling the first radio module 11 to establish a radio connection 30 between the first radio module 11 and the connection partner 2.

[0055] E Check whether a radio connection 30 has been established between the first radio module 11 and the communication partner 2; if no radio connection 30 has been established, proceed to step F; if a radio connection 30 has been established, proceed to step G. F Check whether the predetermined third time period for establishing the radio connection 30 has expired; if the third time period has expired, proceed to step C; if the third time period has not yet expired, proceed to step D.

[0056] G. Comparison of the signal strength of the radio link 30 detected by the first radio module 11 between the first radio module 11 and the link partner 2, in particular as an RSSI value, with the signal threshold 31, which corresponds to a predetermined distance to the link partner 2 of, for example, 3 m and is further exemplified as -70 dBm; if the signal strength is less than the signal threshold 31, proceed to step H; if the signal strength is greater than the signal threshold 31, proceed to step J

[0057] H Controlling the first radio module 11 to transmit a request to the communication partner 2 to determine the distance between the key 1 and the communication partner 2, wherein the request signals to the communication partner a low priority for determining the distance (ranging) because the access request of user 2 to the vehicle is unlikely.

[0058] J Evaluating or checking the sensor signal 20 for the occurrence of the predetermined signal pattern 23 within the first predetermined time interval; if the signal pattern 23, i.e., for example, a predetermined number of steps taken by user 3, is not detected or registered within the first predetermined time interval, proceed to step K; if the signal pattern 23 is detected or registered within the first predetermined time interval, proceed to step L; K Controlling the first radio module 11 to transmit a request to the communication partner 2 to determine the distance between the key 1 and the communication partner 2, the request signaling to the communication partner a medium priority for determining the distance (ranging), since the access request of user 2 to the vehicle is (medium / average) likely.

[0059] L Controlling the first radio module 11 to transmit a request to the communication partner 2 to determine the distance between the key 1 and the communication partner 2, wherein the request assigns a high priority to the communication partner for determining the

[0060] Distance (ranging) is indicated, as user 2's desire to access the vehicle is very likely.

Claims

Patent claims 1. Electronic key (1) for authentication with a communication partner (2) and determination of the distance to the communication partner (2), wherein the key (1) comprises a control module (10), a first radio module (11) for establishing a radio connection (30) with the communication partner (2) enabling information exchange, a second radio module (12) for determining the distance to the communication partner (2), and a motion sensor (13) configured to generate a sensor signal (20) dependent on movement of the electronic key (1), wherein the first radio module (11) is switchable between an active state and an inactive state and is configured to determine a signal strength of the radio connection (30), characterized in that the control module (10) is configuredto compare the sensor signal (20) with a motion threshold (21), to switch the first radio module (11) from its inactive state to its active state and to initiate the radio link (30) when the sensor signal (20) is greater than the motion threshold (21), wherein the control module (10) is configured to compare the signal strength with a signal threshold (31), to initiate the first radio module (11) by sending a low-priority (H) distance determination request to the connection partner (2) via the radio link (30) when the signal strength is less than the signal threshold (31), and to check the sensor signal (20) for a predetermined signal pattern (23) when the signal strength is greater than the signal threshold (31). wherein the control module (10) is configured to send a medium priority (K) distance determination request to the communication partner (2) via the radio link (30) if the predetermined signal pattern (23) is not registered within a predetermined first time interval, and to send a high priority (L) distance determination request to the communication partner (2) via the radio link (30) if the predetermined signal pattern (23) is registered within the first predetermined time interval.

2. Electronic key according to claim 1, wherein the motion sensor (13) is an acceleration sensor and / or the motion limit (21) is an acceleration limit.

3. Electronic key according to claim 1 or 2, wherein the control module (10) is provided as a separate circuit or is integrated into the first radio module (11) or into the second radio module (12) or the motion sensor (13) is integrated and / or wherein the key (1) has a clock generator which is integrated into the first radio module (11) for establishing the radio connection (30) or is connected to the first radio module (11) in terms of signaling and is also connected to the control module (10) in terms of signaling.

4. Electronic key according to one of the preceding claims, wherein the signal pattern (23) is a signal from the motion sensor (13) is a signal generated during a sequence of several successive steps by a user (3) carrying the key (1).

5. Electronic key according to any of the preceding claims, wherein the control module (10) is switchable between an active state and an inactive state and is designed to switch from the inactive state to the active state upon receiving the sensor signal (20).

6. Electronic key according to one of the preceding claims, wherein the second radio module (12) is switchable between an active state and an inactive state and the control module (10) is configured to switch the second radio module (12) from its inactive state to its active state when the first radio module (11) has transmitted a high priority (L) distance determination request to the communication partner (2) or when the first radio module (11) has received a distance determination request from the communication partner (2).

7. Electronic key according to one of the preceding claims, wherein the control module (10) is configured to switch the first radio module (11) and / or the second radio module (12) and / or the control module (10) from its respective active state to its respective inactive state when the motion sensor (13) does not detect any movement within a predetermined second time period and / or when the first radio module (11) does not establish a radio connection (30) with the connection partner (2) within a predetermined third time period.

8. Method for energy-saving operation of an electronic key for authentication vis-à-vis a communication partner (2) and determination of the distance to the communication partner (2), which has a control module (10), a first radio module (11) for establishing a radio communication enabling information exchange The first radio module (11) has a connection (30) with the connection partner (2), a second radio module (12) for determining the distance to the connection partner, and a motion sensor (13) configured to generate a sensor signal (20) dependent on a movement of the electronic key (1), wherein the first radio module (11) is switchable between an active state and an inactive state and is configured to determine a signal strength of the radio connection (30), characterized in that the method comprises at least the following steps: a. comparison of the sensor signal (20) with a movement threshold (21), wherein the first radio module (11) switches from its inactive state to its active state and the radio connection (30) is initiated when the sensor signal (20) is greater than the movement threshold (21); b.Comparison of the signal strength with a signal threshold (31), wherein the first radio module (11) is controlled to send a low-priority (H) distance determination request to the communication partner (2) via the radio link (30) when the signal strength is less than the signal threshold (31), and wherein the sensor signal (20) is checked for a predetermined signal pattern (23) when the signal strength is greater than the signal threshold (31); c. Controlling the first radio module (11) to send a medium-priority (K) distance determination request to the communication partner (2) via the radio link (30). if the predetermined signal pattern (23) is not registered within a predetermined first time interval, and the first radio module (11) is activated to send a high priority (L) distance determination request to the communication partner (2) via the radio link (30) if the predetermined signal pattern (23) is registered within the predetermined first time interval.

9. Method according to the preceding claim, wherein the second radio module (12) is switchable between an active state and an inactive state, and the method further comprising the step: d. switching the second radio module (12) from its inactive state to its active state when the first radio module (11) has transmitted the request to determine the distance with high priority (L) to the communication partner (2) or when the first radio module (11) has received a request to determine the distance from the communication partner (2).

10. Method according to one of the two preceding claims, wherein the control module (10) is switchable between an active state and an inactive state, and the method further comprising the step: e. Switching the control module (10) from the inactive state to the active state upon receiving the sensor signal (20).

11. Method according to any one of the preceding claims 8 to 10, further comprising the step: f. switching the first radio module (11) and / or the second radio module (12) and / or control module (10) from the respective active state to the respective inactive state if the motion sensor (13) does not detect any movement within a predetermined second time period and / or if the first radio module (11) does not establish a radio connection (30) with the connection partner (2) within a predetermined third time period.

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