METHOD FOR SENSOR UNIT SYNCHRONIZATION

Description

[0001] The invention relates to a method for synchronizing sensor units for the simultaneous execution of a measurement at a defined measurement time at different locations by at least one first and one second sensor unit. Extensive investigations of the properties of technical equipment, assemblies, or environments often require the use of several sensors that are arranged at different geographical locations within the equipment, on the assembly, or in the environment, but which record a measurement simultaneously. An example of such an investigation is vibration measurements on a relatively large pump assembly, such as a so-called split-case pump, which requires the use of several sensors at different points on the pump assembly.The measurement data from the different sensors must be recorded at the same time and with as identical a time base as possible in order to create a good basis for further analysis, such as evaluation by spectral analysis.

[0002] To conduct such an investigation, it is common practice to set up complex measurement equipment at the site of the device, the unit, or its environment. This equipment includes at least a central control and evaluation unit and the aforementioned multiple sensors. The control and evaluation unit has a multitude of analog and / or digital inputs, with analog-to-digital converters typically following the analog inputs. The sensors are connected to the control and evaluation unit via appropriate lines or cables, or fiber optics for longer distances, and simultaneously transmit corresponding measurement signals. Due to the simultaneous reception of these signals, the control and evaluation unit assigns a uniform time base to these signals.

[0003] It goes without saying that such a control and evaluation unit for carrying out the investigation is expensive if it is to process multiple inputs from a large number of sensors simultaneously. However, the number of usable sensors is regularly limited, meaning it is not entirely flexible in its selection. Modular concepts for measurement systems are known in which the corresponding control and evaluation unit is scalable or expandable with additional measurement modules to allow the connection of further sensors. A disadvantage of this concept, however, remains that the end customer pays for unused hardware if they do not use all sensors or inputs.

[0004] US 2021 / 0116902 A1 discloses a system comprising sensors managed by a management device. The management device communicates with one or more sensors over a network. Each sensor maintains a common time, which can be provided to each sensor by its operating system. The system provides a measurement command. The measurement command can be an electrical signal that instructs the sensors to perform an action, such as synchronous measurements on one or more machines. The measurement command can include a time (e.g., an initial time value) at which the action is to be performed. The management device sends the measurement command to the sensors over the network.

[0005] US 2018 / 0270548 A1 discloses a cloud-connected sensor system in which remote sensors are wirelessly connected to a sensor facility. The cloud-based processing service includes at least one processor programmed with control software or scripts containing measurement commands that manipulate the facility's internal hardware configuration to acquire raw data from wired and / or remote sensors. These measurement commands include sensor-specific DAS settings that determine how the facility's internal hardware components operate to perform measurements or acquire raw data using a specific wired or wireless sensor connected to the facility. The DAS settings are transmitted to the facility via a real-time persistent connection (e.g., an MQTT tunnel) between the cloud service and the facility.The cloud-based processing service broadcasts or publishes a specific topic, and the facility's hardware API subscribes to the broadcast / published topic. Topics can generate measurement commands. These commands are sent to various sensors. User interaction occurs via a handheld device. The command settings specify the duration and timing of sample stimulation prior to data acquisition.

[0006] The object of the present invention is to create a simple, expandable and cost-effective testing method with a virtually unlimited number of sensor units, which are controlled in such a way that they perform a measurement simultaneously at a defined measurement time.

[0007] This problem is solved by a method with the features of claim 1. Advantageous further developments are specified in the dependent claims and are explained below.

[0008] According to the invention, a method is proposed in which the sensor units each have their own real-time clock and a radio module with an individual identifier for communication via a radio network, in particular a cellular mobile network and the Internet with a remote messaging service, and the following steps are carried out: An operator transmits a measurement order to the messaging service to perform the measurement at the defined measurement time. The messaging service then transmits a first message containing the measurement time to the first sensor unit and a second message containing the measurement time to the second sensor unit. The first and second sensor units then execute the measurement order at the defined measurement time by recording measurement data. Finally, the first and second sensor units transmit the recorded or processed measurement data via the messaging service to the operator and / or an evaluation center.

[0009] The basic idea of ​​the present invention is to use sensor units for an investigation or measurement that acquire measurement data independently of one another, both temporally and organizationally. This data is then transmitted to the cloud and evaluated there, i.e., at any location remote from the measurement site. For this to work, the sensor units must be triggered simultaneously to ensure coherent measurement data. This synchronization of measurements is ensured by the messaging service on the one hand and the real-time clock integrated into the sensor units on the other. The sensor units can be small and inexpensive (low-cost sensors), yet the multitude of different measurement data, which provide a picture of the condition of the equipment, system, or environment from multiple perspectives, still enables high-quality analysis.

[0010] A particular advantage of the invention lies in the flexible expandability of the installation. Depending on requirements, a different number of sensors can be used.

[0011] Insofar as the invention refers to "simultaneous" or "same time" for the start of the measurement, it should be noted that these terms do not refer to a nanosecond- or microsecond-accurate simultaneous execution of the measurement by the sensor units, as such precise synchronization of the real-time clocks of the sensor units is not possible. Rather, the simultaneous execution of the individual measurements is to be understood as meaning that they can also begin at different times within a time interval of at most one second. It is assumed that during this time interval, a state detected by the sensor units or a measured quantity does not change or changes only insignificantly for the measurement purposes. A time difference of at most one second is therefore still to be considered "simultaneous" within the scope of the method.

[0012] The measurement can involve one or more measurands. These measurands can be the same or different. Accordingly, a sensor unit can have one or more sensors to detect one or more measurands. For example, a sensor unit could be a combination of three accelerometers, each measuring acceleration in the x, y, or z direction. In this case, one sensor unit detects three measurands of the same type. Another sensor unit could, for example, measure current and voltage, and possibly also electrical power. In this case, too, one sensor unit detects more than one measurand, but of different types.

[0013] A sensor unit may have one or more integrated (internal) sensors that perform the measurement. Alternatively or cumulatively, a sensor unit may have one or more standard interfaces (0-10V, 4-20mA, 1-wire, PWM) to which one or more (external) sensors are connected.

[0014] Alternatively or cumulatively, a sensor unit can include preprocessing of the measurement data, particularly in the form of filters or calculations, such as low-pass filtering to reduce noise, moving average calculation, or Fourier transformation. These preprocessed sensor signals then constitute the measurement data to be transmitted. Thus, the measurement data to be transmitted is not in its recorded (raw) form, but rather in a processed form, as provided by the sensor unit.

[0015] The measurement can be performed over a short or a longer period. From the defined measurement time, one or more measured values ​​are recorded by the sensor units for a defined period and subsequently sent to the cloud. The sampling times of the sensor units can be identical or different, as the recorded values ​​share a common, known start time corresponding to the measurement time.

[0016] In common parlance, the term "cloud" refers to a distributed arrangement of computers, or more precisely, servers, connected to each other via the internet. The messaging service is operated on such a server, and is therefore part of the cloud.

[0017] The evaluation center can also be a cloud server or run on one, connected to the messaging service via the internet. Alternatively, the evaluation center can be located on a server connected to the messaging service only via a local area network (LAN). In another configuration, the evaluation center can run on the same server as the messaging service, so that communication between the evaluation center and the messaging service takes place entirely within the server.

[0018] The wireless network can be a so-called LoRaWAN (Long Range Wide Area Network), which was specifically developed for the Internet of Things (IoT) and is a network standard for energy-efficient transmission and reception of data over long distances. Each radio module is assigned a unique identifier by which it can be identified within the network. Time information is available in a LoRaWAN to synchronize the real-time clocks of the sensor units. Alternatively, a time server can be queried via the LoRaWAN.

[0019] For communication with the cellular mobile network, the radio module of each sensor unit includes a physical or virtual subscriber identification module, commonly referred to as a SIM card. This module is assigned an identifier that uniquely identifies the subscriber identification module, and thus the sensor unit, within the mobile network and enables access to the network's resources, particularly its data network, so that the respective sensor unit can access the internet via the mobile network.

[0020] Preferably, the radio module uses the NB-IoT (Narrowband Internet of Things) radio standard. This is a low-power wide-area network (LPWAN) radio standard developed by 3GPP for mobile devices and services and defined in the 3GPP Release 13 (LTE Advanced Pro) specification from June 2016. The advantages of NB-IoT include low cost, long battery life for the sensor unit, and high connection density. NB-IoT uses a subset of the LTE standard but limits the bandwidth to a single narrow band of 200 kHz.

[0021] The message switching service is a communication platform that can be used by a large number of participants (clients), with the sensor units, the operator, and the evaluation center each being considered a participant. The message switching service can also be viewed as a broker that forwards messages between the participants.

[0022] The operator can be considered the "master," whereas the sensor units are "slaves." The operator can be software that provides a communication interface to the messaging service, enabling a person or a computer program to send and receive messages to and from the messaging service, in particular to create measurement orders and receive measurement data.

[0023] Preferably, communication via the messaging service takes place exclusively within topic-specific messaging channels, where participants can post messages on a particular topic and which can then be listened to by other participants. The messaging service publishes the content of a posted message in the messaging channel by transmitting it to interested participants.

[0024] Thus, it can be stipulated that the defined measurement time is published by the messaging service in a specific messaging channel identified in the (posted) measurement order. The measurement order is therefore a message with content intended for that messaging channel.

[0025] Preferably, the measurement request not only identifies the message channel, e.g., "measurement," but also specifies its content by using a parameter such as "time." This allows interested participants to listen specifically to certain content within messages posted in that channel.

[0026] Ideally, before transmitting the measurement order, the operator should first log in to the messaging service to record the measurement time. Only after logging in does a dedicated client-server relationship exist between the operator and the messaging service. The same procedure can be implemented for the sensor units.

[0027] It is possible for the participants, i.e., the operator, evaluation center, and first and / or second sensor unit, to remain permanently connected to the messaging service or to be connected only temporarily. The latter saves power and is preferable for increasing the service life of the sensor units, which are preferably battery- or accumulator-powered.

[0028] To save energy on the one hand, and on the other hand not to miss any messages intended for them, such as measurement orders, the first and second sensor units each register with the messaging service independently and regularly after a reporting interval has elapsed. For example, the reporting interval can be between 30 minutes and 24 hours, preferably between 1 and 6 hours.

[0029] To receive messages on the messaging channel, the first and second sensor units can each subscribe to the messaging channel after registering with the messaging service.

[0030] To prevent a sensor unit from missing a message intended for it due to only temporarily registering with the message service, the operator can transmit the measurement order at a time that is more than the length of the message interval before the defined measurement time. This ensures that the sensor units are informed of the measurement time.

[0031] It can be stipulated that the measurement time is only stored on the messaging service for a specific period and then deleted. This reduces the storage requirement for the message content. However, the measurement time should be stored on the messaging service for the specified duration so that it can be made available to subsequently registered sensor units. This duration should be longer than the reporting interval of the sensor units to ensure the transmission of the measurement time to them.

[0032] Advantageously, the transmission of the first message from the messaging service to the first sensor unit occurs only, or only when, the first sensor unit has registered with the messaging service and subscribed to the messaging channel. Similarly, the transmission of the second message from the messaging service to the second sensor unit preferably occurs only, or only when, the second sensor unit has registered with the messaging service and subscribed to the messaging channel.

[0033] As already mentioned, it is planned that the first and second sensor units will each independently log off from the messaging service after a registration period has expired in order to save energy.

[0034] After the measurement task has been completed, the first sensor unit can transmit at least one initial measurement message containing initial measurement data to the messaging service, and the second sensor unit can transmit at least one second measurement message containing additional measurement data to the messaging service, independently of each other and possibly at different times. If the measurement data, due to its size or volume, does not fit into a single measurement message, it can be distributed across two or more measurement messages.

[0035] The transmission of the first and second measurement messages ideally only occurs once the first and second sensor units have each independently re-registered with the messaging service. In other words, a short time elapses after the measurement before the measurement data is transmitted to the evaluation center.

[0036] The first measurement data can then be published by the messaging service in a specific messaging channel identified in the first measurement message. Similarly, the second measurement data can be published by the messaging service in a specific messaging channel identified in the second measurement message. It should be noted that the messaging channels do not have to be identical. Furthermore, they can also differ from the messaging channel in which the measurement time was published. Nevertheless, for the sake of simplicity and thematic relevance, it is advisable that the messaging channels identified in the measurement messages refer to the same messaging channel, and that this channel is identical to the one specified in the measurement order.

[0037] After receiving the first and second measurement messages, the messaging service can transmit at least one first publication message containing the first measurement data and at least one second publication message containing the second measurement data to the evaluation center and / or the operator. If the measurement data, due to its size or volume, does not fit into a single publication message, it can be distributed across two or more publication messages, which are then reassembled by the recipient, i.e., the evaluation center or the operator, to form the original measurement data.

[0038] In one implementation variant, the operator can include the evaluation center. In another implementation variant, the evaluation center can be located downstream of the operator, so that the operator forwards the measurement data to the evaluation center.

[0039] Preferably, the transmission of the first and second measurement data takes place when or as soon as the evaluation center or the operator has registered with the messaging service and subscribed to the messaging channel identified in the respective measurement message.

[0040] After receiving the first and second measurement data, these are evaluated together in the evaluation center and / or by the operator.

[0041] The measurement data mentioned can either be raw data acquired by a corresponding sensor of the first or second sensor unit S1, S2, or alternatively be pre-processed data resulting from the raw data through filtering, smoothing, averaging or transformation, such as a Fourier transform.

[0042] In order for the sensor units to calibrate their respective real-time clocks, it may be provided that they synchronize these with a time signal or time provided to them by a time signal transmitter as a broadcast signal, the mobile network, the messaging service, an NTP server, and / or a global navigation satellite system (GPS).

[0043] It should also be noted that the time signal or the real-time clock does not necessarily have to be specified in the format hours, minutes, and seconds. Other time formats, such as a simple counter, can also be used.

[0044] To execute the previously described process steps, the message brokering service can be an MQTT broker, for example according to the MQTT version 5.0 OASIS standard of March 7, 2019. MQTT (Message Queuing Telemetry Transport) is an open network protocol for machine-to-machine communication that enables the transmission of telemetry data in the form of messages between devices, despite high latency or limited networks.

[0045] The method according to the invention can be used particularly for carrying out and evaluating vibration measurements on a centrifugal pump unit.

[0046] Further features, properties, effects, and advantages of the invention are explained in more detail below with reference to exemplary embodiments and the accompanying figures. The reference numerals in the figures retain their meaning from figure to figure. In the figures, reference numerals always denote the same or at least equivalent components, areas, directions, or locations.

[0047] It should be noted that, within the context of this description, the terms "exhibit," "comprise," or "include" in no way exclude the presence of other characteristics. Furthermore, the use of the indefinite article for an object does not preclude its plural form.

[0048] Figuren 1a and 1b show an exemplary temporal sequence of the communication according to the invention via a message exchange service for synchronizing a measurement performed by two sensor units.

[0049] Message service 1 is an MQTT broker to which virtually any number of clients can register in a server-client relationship to send or receive messages within a specific messaging channel. Message service 1 acts as the server in this case. It is connected to the internet and can therefore be reached. In the example shown here, there are three clients: Client A, an operator 3, who requests that a measurement be taken at a specified time tx by a first sensor unit S1 and a second sensor unit S2, which, in relation to message service 1, constitute the other two clients B and C. In another implementation, many more sensor units could be used, which would then act as clients of message service 1.

[0050] Operator 3 can be understood as user-operated software running on an end device such as a computer (personal computer), tablet, or smartphone, where the end device is connected to the internet to establish a connection with the messaging service 1. To do this, Operator 3 sends a connection request ("CONNECT") to the messaging service 1, which is acknowledged by the latter with a subsequent connection acceptance ("CONNACK"). After this, Operator 3 is registered with the messaging service 1. It remains registered for the duration of the procedure under consideration.

[0051] After logging in, Operator 3 subscribes to a first message channel 6 "Measurement / Vibration" (SUBSCRIBE) to receive messages containing measurement data. Furthermore, in a second message channel 6 "Measurement / Time" (PUBLISH), he places a measurement order 2, the content of which is the measurement time tx at which the sensor units S1, S2 are to perform the measurement. The task of the message service 1 is to publish the messages intended for a specific message channel 6 by immediately delivering the corresponding messages 4, 5, 9, 10 to all clients A, B that have subscribed to this message channel 6.

[0052] At the time operator 3 records the measurement time tx, no other client has yet subscribed to the second message channel 6 in this example. In this case, message service 1 would delete the measurement time tx. However, to prevent or at least delay this, the measurement time tx is marked with the attribute "Retain" and thus held by message service 1 for a defined period of time for later publication.

[0053] The sensor units S1 and S2 are located at different points, i.e., spatially separated from each other. Each is configured to perform a vibration measurement on a centrifugal pump unit to which it is attached. Each unit has its own real-time clock to execute the measurement at time tx.

[0054] Furthermore, sensor units S1 and S2 each possess a radio module with a unique identifier in the form of a virtual or physical SIM card, enabling them to connect to a cellular mobile network and thereby gain access to the internet, and thus to the messaging service 1. In this case, the mobile network is exemplified as an LTE (Long Time Evolution) network, i.e., a fourth-generation mobile network. Sensor units S1 and S2 connect to the mobile network via a corresponding base station (NodeB, eNodeB) and the connected control unit (RNC - Radio Network Controller) of the mobile network in a generally known manner, according to the specifications of the standardization organization. 3rd Generation Partnership Project (3GPP).

[0055] To connect to the mobile network, the first sensor unit S1 first sends an "Attach Request" to the mobile network, which is answered by the network with a confirmation "Attach Accept", so that the first sensor unit S1 gains access to the internet via the mobile network.

[0056] The mobile network transmits system information (System Information Broadcast) to all its registered subscribers, including a time signal. This signal is used by the first sensor unit S1 to update its internal real-time clock, i.e., to synchronize it with the time information from the mobile network. Alternatively or additionally, the first sensor unit S1 can query a time server on the internet (NTP server), receive the broadcast signal from a time signal transmitter, and / or receive the time from a global navigation satellite system (GPS), and synchronize its real-time clock with the time thus obtained. This time update can occur repeatedly, particularly on a regular basis.

[0057] To register with message service 1, which is done via the mobile network and the internet, the first sensor unit S1, like operator 3 before it, sends a connection request "CONNECT" to message service 1, which is acknowledged by the latter with a subsequent connection acceptance "CONNACK". After this, sensor unit S1 is also registered with message service 1.

[0058] The first sensor unit S1 then subscribes to the second message channel "Measurement / Time" (SUBSCRIBE) to be informed about any measurement order 2 from operator 3. This prompts the messaging service 1 to make the measurement time tx of measurement order 2, previously stored in the second message channel "Measurement / Time" and retained due to the "Retain" attribute, public by transmitting an initial message 4 containing the measurement time tx to the first sensor unit S1. This occurs—as before—via the internet and the mobile network. The first sensor unit S1 receives the initial message 4 and stores the measurement time tx. To conserve energy, the first sensor unit S1 then disconnects from the messaging service 1 by transmitting a corresponding "DISCONNECT" message to the messaging service 1.This remains unacknowledged because the first sensor unit S1 is subsequently no longer reachable.

[0059] To save even more energy, the first sensor unit S1 can also disconnect from the mobile network. This is in Fig. 1a however, it is not shown.

[0060] The procedural steps described for the first sensor unit S1 are also performed for the second sensor unit S2. To connect to the mobile network, the second sensor unit S2 also first sends an "Attach Request" to the mobile network, which is answered with an "Attach Accept" confirmation, thus granting the second sensor unit S2 access to the internet via the mobile network. The time transmitted by the mobile network is also used by the second sensor unit S2 to update its internal real-time clock, i.e., to synchronize it with the time information from the mobile network. Alternatively or additionally, the first sensor unit S1 can query a time server on the internet (NTP server), receive the broadcast signal from a time signal transmitter, and / or receive the time from a global navigation satellite system (GPS), and synchronize its real-time clock with the time thus obtained.The aforementioned update can be repeated, especially on a regular basis.

[0061] To register with message service 1, the second sensor unit S2 sends a connection request "CONNECT" to message service 1, which is acknowledged by the latter with a subsequent connection acceptance "CONNACK". Sensor unit S2 is then registered with message service 1. It subsequently subscribes to the second message channel "Measurement / Time" (SUBSCRIBE) to be informed of any measurement order 2 from operator 3. Message service 1 then provides the measurement time tx of the measurement order to the second sensor unit S2, which was previously published in the second message channel "Measurement / Time" and is still retained due to the "Retain" attribute, by transmitting a second message 5 containing the measurement time tx to the second sensor unit S2. This also occurs via the internet and the mobile network.The second sensor unit S2 receives the second message 5 and stores the measurement time tx.

[0062] To save energy, the second sensor unit S2 also disconnects from message service 1 by transmitting a corresponding "DISCONNECT" message to message service 1. This also remains unacknowledged. To further save energy, the second sensor unit S1 can also disconnect from the mobile network. This is in Fig. 1a however, it is not shown.

[0063] Although these procedural steps concerning the second sensor unit S2 in Figur 1a The chronological order of the steps for the second sensor unit S2, following those of the first sensor unit S1, should be noted as being solely for simplification and clarity. In fact, the process steps for the second sensor unit S2 can also be executed concurrently with those for the first sensor unit S1, since the first and second sensor units S1 and S2 are independent of each other and determine their own connection times with the mobile network and the messaging service 1. This can depend, in particular, on the commissioning time of the respective sensor. Thus, the first and second sensor units S1 and S2 can each independently and regularly register with the messaging service 1 after a reporting interval to be informed of any newly published messages, especially measurement orders 2.Accordingly, the first and second sensor units S1 and S2 can each independently unregister from the message exchange service 1 after the end of a registration period.

[0064] Due to the uncertainty of when one of the sensor units S1, S2 will report to the message service 1, operator 3 transmits the measurement order 2 at a time that is earlier than the defined measurement time tx by more than the length of the reporting interval. Furthermore, the duration for which the measurement time tx is held at the message service 1 is longer than the reporting interval of the sensor units S1, S2. After this duration has elapsed, the measurement time tx is deleted.

[0065] The first and second sensor units, S1 and S2, then wait for the measurement time tx to occur. To do this, they each compare the current time of their internal real-time clock with the measurement time tx and simultaneously start their respective measurements when tx is reached. This is described in Figur 1a This is illustrated by the horizontal dashed line. The first and second sensor units S1, S2 then record measurement data D1, D2, thus executing the original measurement task 2 at the defined measurement time tx.

[0066] The measurement can be performed for a longer or shorter period. It should be noted that the measurement periods of the first and second sensor units S1 and S2 do not necessarily have to be the same length.

[0067] The procedure steps to be carried out after the respective measurement of the first and second sensor units S1, S2 are described in Figur 1bAs shown, it is intended that all sensor units S1 and S2 transmit their measurement data to the message switching service 1, which then forwards it to operator 3.

[0068] To this end, the first sensor unit S1 first reconnects to the messaging service 1 with a corresponding connection request ("CONNECT"), which is acknowledged ("CONNACK"). The first sensor unit S1 then sends an initial measurement message 7, containing the measurement data D1 of the first sensor unit S1, to the messaging service 1 for publication in the first message channel "Measurement / Vibration" ("PUBLISH"), which is identified in the first measurement message 7. The first sensor unit S1 then disconnects from the messaging service 1 ("DISCONNECT"). If the measurement data D1 is too large for a single measurement message 7, the first sensor unit S1 can also split the measurement data D1 and transmit it to the messaging service 1 in multiple measurement messages 7.

[0069] Since Operator 3 has subscribed to the first message channel, the message service 1 publishes the measurement data D1 of the first sensor unit S1 by sending Operator 3 a first publication message 9 containing the measurement data D1. Operator 3 extracts the measurement data D1 from the first publication message 9 and stores it.

[0070] Similarly, the second sensor unit S2 again connects to the messaging service 1 with a corresponding connection request "CONNECT", which is acknowledged (CONNACK). The second sensor unit S2 then sends a second measurement message 8, containing the measurement data D2 of the second sensor unit S2, to the messaging service 1 for publication in the first message channel "Measurement / Vibration" (PUBLISH), which is identified in the second measurement message 8. The second sensor unit S2 then disconnects from the messaging service 1 (DISCONNECT). If the measurement data D2 is too large for a single measurement message 8, the second sensor unit S2 can also split the measurement data and transmit it to the messaging service 1 in multiple measurement messages 8.

[0071] The message service 1 forwards the measurement data D2 of the second sensor unit S2, published in the second measurement message, to operator 3 by sending him a second publication message 10 containing the measurement data D2. Operator 3 extracts the measurement data D2 from the second publication message 10 and stores it. He can then perform a combined analysis based on the common time base of the measurement data D1 and D2.

[0072] In another implementation variant, the combined analysis can be performed in an evaluation center. Operator 3 can forward the measurement data to this center. Alternatively, the evaluation center can also be a client of the messaging service 1, register with it accordingly, subscribe to the first messaging channel, and thus receive the measurement data D1, D2 after it has been published in the first messaging channel by the corresponding sensor unit S1, S2.

[0073] It should be noted that the foregoing description is given merely as an example for illustrative purposes and in no way limits the scope of protection of the invention. Features of the invention that are indicated as "may", "exemplary", "preferred", "optional", "ideal", "advantageous", "if applicable", "suitable" or the like are to be considered purely optional and likewise do not limit the scope of protection, which is exclusively defined by the claims.

Claims

1. Method for the synchronisation of sensor units (S1, S2) for simultaneously taking a measurement at a defined time of measurement (tx) at different locations, using at least one first and one second sensor unit, respectively having their own real-time clock and a wireless module with an individual identifier for communication via a wireless network, notably a cellular mobile communications network, and the Internet with a remote messaging service (1), comprising the steps of - transmitting, by an operator (3), a measurement order (2) to take the measurement at the defined time of measurement (tx) to the messaging service (1), - transmitting a first message (4) comprising the time of measurement (tx) from the messaging service (1) to the first sensor unit (S1) and a second message (5) comprising the time of measurement (tx) from the messaging service (1) to the second sensor unit (S2), - executing the measurement order (2) by the first and second sensor units (S1, S2) at the defined time of measurement (tx) in that the first and second sensor units (D1, D2) respectively record measurement data, and - transmitting the respective measurement data (D1, D2) in the recorded or an edited form from the first and second sensor units (S1, S2) via the messaging service (1) to the operator (3) and / or to an evaluation centre, in which the first and second sensor units (S1, S2) respectively and independently of each other regularly check in with the messaging service (1) after the end of a reporting interval and, respectively and independently of each other, check out with the messaging service (1) after the end of a check-in interval.

2. Method according to claim 1, characterised in that the defined time of measurement (tx) is published by the messaging service (1) on a defined message channel (6) identified in the measurement order (2).

3. Method according to claim 2, characterised in that the operator (3) initially checks in with the messaging service (1) in order to record the time of measurement (tx).

4. Method at least according to claim 2, characterised in that the first and second sensor units (S1, S2), respectively after checking in with the messaging service (1), subscribe to the message channel (6).

5. Method according to one of the preceding claims, characterised in that the operator (3) transmits the measurement order (2) at a point in time that is more than the length of the reporting interval earlier than the defined time of measurement (tx).

6. Method according to one of the preceding claims, characterised in that the time of measurement (tx) is held available by the messaging service (1) for only a certain period of time that is longer than the reporting interval of the sensor units (S1, S2) and being erased after that time ends.

7. Method at least according to claim 2, characterised in that the transmission of the first message (4) takes place only when or after the first sensor unit (S1) has checked in with the messaging service (1) and has subscribed to the message channel (6), and / or the transmission of the second message (5) taking place only when or after the second sensor unit (S2) has checked in with the messaging service (1) and has subscribed to the message channel (6).

8. Method according to one of the preceding claims, characterised in that the first sensor unit (S1) transmits a first measurement message (7) comprising at least a first set of measurement data (D1) to the messaging service (1) and the second sensor unit (S2) transmitting a second measurement message (8) comprising at least a second set of measurement data (D2) to the messaging service (1) after execution of the measurement order (2) .

9. Method according to claim 8, characterised in that the respective transmission of the first and second measurement messages (7, 8) takes place only after the first and second sensor units (S1, S2), respectively and independently of each other, have checked in with the messaging service (1).

10. Method according to claim 8 or 9, characterised in that the first set of measurement data (D1) is published by the messaging service (1) in a defined message channel (6) identified in the first measurement message (7) and / or the second set of measurement data (D2) being published by the messaging service (1) in a defined message channel (6) identified in the second measurement message (8).

11. Method according to one of the claims 8 through 10, characterised in that the messaging service (1) transmits respectively at least one first publication message (9) comprising the first set of measurement data (D1) and at least one second publication message (10) comprising the second set of measurement data (D2) to the evaluation centre and / or to the operator (3), notably when or as soon as they have checked in with the messaging service (1) and subscribed to the message channel (6) identified in the respective measurement message (7).

12. Method according to one of the preceding claims, characterised in that the first and second sets of measurement data (D1, D2) are jointly evaluated in the evaluation centre and / or by the operator (3).

13. Method according to one of the preceding claims, characterised in that the sensor units (S1, S2) synchronise their respective real-time clock with a time signal provided to them by a time signal transmitter as a radio signal, by the mobile communications network, the messaging service (1), an NTP server and / or a global navigation satellite system (GPS).