FI20206020A1 - Synchronizing sensor measurements - Google Patents

Abstract

This document discloses a solution for synchronizing measurements. According to an aspect, there is provided a method for synchronizing sensor devices, comprising: broadcasting (200, 202, 204, 206), by a controller, a periodic beacon signal comprising a field for a frame count value; transmitting (203) , by the controller to a plurality of sensor devices, a message comprising a specific frame count value for triggering start of measurements; upon detecting, by the plurality of sensor devices, the specific frame count value in a received beacon signal, triggering (208) the start of said measurements.

Description

SYNCHRONIZING SENSOR MEASUREMENTS

TECHNICAL FIELD Embodiments described herein relate to sensor devices and, in particular, to synchronizing measurements performed by the sensor devices

TEACHNICAL BACKGROUND In some sensor configurations, it is particularly important that multiple sensor devices performing measurements are synchronized to a common clock, e.g. when the sensor devices are measuring the same object. For example, motion sensors measuring the same object such as machine vibrations should have an accurate time synchronization. In industrial applications, sensor devices are conventionally connected via wired connections. Transition to wireless sensor systems while maintaining accurate synchronization between the sensor devices would be advantageous.

BRIEF DESCRIPTION Some aspects of the invention are defined by the independent claims. Some embodiments of the invention are defined in the dependent claims. The embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention. Some aspects of the disclosure are defined by the independent claims.

BRIEF DESCRIPTION OF THE DRAWINGS 2 In the following the invention will be described in greater detail by S means of preferred embodiments with reference to the accompanying drawings, in O 25 — which © Figure 1 illustrates a sensor system according to an embodiment; Figure 2 illustrates a procedure for synchronizing start of i measurements according to an embodiment; < Figures 3 to 6 illustrates various embodiments for triggering a start of 3 30 measurements in a sensor device; O Figure 7 illustrates a flow diagram of a procedure for a controller apparatus of the sensor system; and Figures 8 and 9 illustrates block diagrams of apparatuses according to some embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS Figure 1 illustrates a measurement system or a sensor system to which embodiments may be applied. The sensor system may include a plurality of sensor devices 110, 112, 114. The sensor devices may be configured to measure the same object 100, e.g. a machine, a structure, a living object, or any other object. In some scenarios, the sensors devices may be arranged to measure different objects. Each sensor device may have wireless communication capability according to one or more wireless communication protocols. The sensor devices may be configured wirelessly and they may transmit at least some of measured measurement data wirelessly for signal processing and analysis.

The sensor device may include sensors of the same type, e.g. motion sensors, or sensors of different types. For example, a set of motion sensors such as accelerometers may be configured to measure motion of a machine, e.g. vibration of an engine. In other embodiments, the same object may be measured by different types of sensors, e.g. a first set of one or more motion sensors and a second set of one or more optical sensors or electrical sensors. Referring to Figure 1, the operation of the sensor devices may be controlled by a controller apparatus 120 disposed such that the sensor devices are within a radio coverage area of the controller apparatus. In some embodiments, the controller apparatus operates as a router, collects the measurement data wirelessly from the sensor devices and forwards the measurement data to a server computer 140 via one or more computer or communication networks 130. The controller apparatus 120 and the server computer may be directly connected to one another in which case the network(s) 130 may be omitted. The server S computer may have signal processing capability for analysing the measurement & data and outputting one or more measurement metrics representing the analysis. N As discussed in Background, accurate synchronization of the wireless 2 sensor devices would be advantageous. A signalling diagram of Figure 2 illustrates E 30 an embodiment where the sensor devices are synchronized to start the o measurements at the same time by employing beacon frames or beacon signals S broadcasted periodically by the controller 120. A beacon signal may comprise a S radio signal carrying the beacon frame. Referring to Figure 2, the procedure for N synchronizing start of the measurements comprises: broadcasting (steps 200, 202, 204, 206), by the controller 120, a periodic beacon signal comprising a field for a frame count value N; transmitting (step 203), by the controller to the sensor devices 110 to 114, a message comprising a specific frame count value N+3 for triggering start of measurements; and upon detecting, by the sensor devices 110 to 114, the specific frame count value N+3 in a received beacon signal (received in step 206), triggering the start of said measurements (block 208).

An advantage in synchronizing the start of the measurements to the periodic beacon is that all the devices receive the beacon at substantially the same time instant. This overcomes the problem of conventional solutions where internal clocks of the sensor devices are repeatedly synchronized with a master clock, — which results in signalling and is also unreliable. The unreliability is caused by internal clock drifts in the sensor devices. The beacon reception time serves as a common time reference that is the same for all sensor devices, regardless of their internal clock values.

In an embodiment, the purpose of the measurements is to measure the object, as described above. The measurements may distinguish from any measurements related to the radio communication between the controller and the sensor devices, e.g. from radio channel measurements. When considering the measurements from the perspective of communication protocol layers, the measurements may operate on the highest application layer, thus distinguishing from the lowest, physical layer measurements related to radio communications.

In an embodiment, the frame count value increments as the periodic beacon signals are being broadcasted. The increment may follow the periodicity, i.e. increment in every beacon period. Other types of increments are equally possible, as long as the incrementation enables the sensor devices to follow the incrementation and anticipate the beacon frame that will trigger the o measurements. The frame count value may even be a flag or a similar information AN element that is not as such incrementing. For example, the controller may switch 5 the flag to indicate measurements a certain number of beacon frames before the a beacon frame triggering the measurements. However, even in such cases the flag A 30 — together with the knowledge of the certain number of beacon frames would serve E as a frame count value. o In an embodiment, the sensor devices are configured to acknowledge S (steps 210, 212, 214) the reception of the message transmitted in step 203. In such N embodiments, the message may be a unicast message addressed to an individual N 35 sensor device or a multicast message addressed to a group of sensor devices. The message may even be a broadcast message, e.g. a beacon frame having the field for one field for the frame count value of the beacon frame and another field for the specific frame count value of a future beacon frame that will trigger the measurements. The controller may transmit the message sufficiently early with respect to the beacon frame triggering the start of the measurements to enable the sensor devices to acknowledge the reception of the message. The controller may determine the timing of the message with respect to the beacon frame triggering the start of the measurements on the basis of the number of sensor devices in the system. The message may be transmitted earlier when the number of acknowledging sensor devices is higher, and the message may be transmitted later — when the number of acknowledging sensor devices is lower. Accordingly, the length of the time interval between the message and the beacon frame triggering the start of the measurements may be proportional to the number of sensor devices in the system.

In an embodiment, the timing of the message with respect to the beacon — frame triggering the start of the measurements is sufficiently early to enable the controller to ensure that all the sensor devices have detected and acknowledged the message and to carry out back-up procedures in a case where acknowledgment from one or more sensor devices has not been received by the controller. The back- up procedures may include retransmission of the message at least to the one or more sensor devices that have not successfully acknowledged the reception of the message. The back-up procedures may also include making a decision of whether or not to proceed with the measurements even in the case where the one or more sensor devices that have not successfully acknowledged the reception of the message. The controller may choose to cancel the triggering or to proceed with the triggering. In case the controller chooses to cancel the triggering, the controller o may transmit a cancellation message at least to the sensor devices that had AN acknowledged the reception of the earlier message. In case the controller chooses 5 to proceed with the triggering, the controller may attempt the retransmission of a the message indicating the frame count value until the timing of the beacon frame A 30 having the frame count value. In an embodiment, the sensor devices are configured E to transmit at least some of the measurement data measured in block 208 o wirelessly to the controller 120 (step 220). In other embodiments, the S measurement data is collected from the sensor devices in a wired manner. N In an embodiment, the sensor devices suspend all transmissions for the N 35 duration of the periodic beacon signal. In this manner, the beacons are protected from collisions with other transmissions in a radio channel, thus improving the beacon detection probability in the sensor devices. As described in some embodiments below, a guard time may precede the beacon transmission. The sensor devices may be configured to switch to a reception mode during the guard time to protect the beacon and to enable detection of the beacon.

5 As described above in connection with Figure 2, some embodiments are realized in the sensor devices 110 to 114 while other embodiments are realized in the controller apparatus 120. The procedure in the sensor device comprises receiving, from the controller, the periodic beacon signal comprising a field for a frame count value incrementing with every beacon signal; receiving, from the — controller, the message comprising the specific frame count value for triggering the start of measurements and, upon detecting the specific frame count value in the received beacon signal, triggering the start of said measurements.

Figure 3 illustrates an embodiment of such a procedure in a sensor device, e.g. one of the sensor devices 110 to 114. Referring to Figure 3, the sensor — device may configure its radio transceiver to receive a radio signal during the beacon guard time or guard period (block 300). In block 302, the sensor device determines the beacon frame count of the next beacon signal and compares the coming beacon frame count with the beacon frame count that will trigger the measurements, i.e. whether or not the coming beacon frame count value eguals — with the beacon frame count value triggering the measurements. If the next beacon frame count is determined on the basis of the comparison to not trigger the measurements, the process may proceed to block 304 where the next beacon period is waited for. Meanwhile, the sensor device may perform other tasks, e.g. access theradio channel to transmit frames, or it may stay idle. If the next beacon frame count is determined on the basis of the comparison to trigger the o measurements, the process may proceed to block 305 where the sensor device N prepares its sensor to perform measurements. Block 305 may comprise activating 5 the sensor(s) of the sensor device, configuration of measurement parameters to a the sensor(s), or even starting the measurements as described in some A 30 embodiments below. Thereafter, upon detecting the beacon in the radio E transceiver in block 306, the sensor device may trigger the start of the o measurements in block 308. Since the sensor device already knows the frame count S value of the beacon signal, it is not even necessary to extract the frame count value N from the beacon frame to trigger the measurements. N 35 It should be appreciated that the steps in the procedure of Figure 3 may be carried out in a different order. For example, the sensor device may carry out block 305 before block 302, e.g. a certain number of beacon frames before the beacon frame triggering the start of the measurements. For example, activation of the sensor(s) may take more time than a single beacon interval and, therefore, block 305 may be carried out in advance so that the sensor(s) is/are ready to measure when the triggering occurs. Similarly, block 302 may precede block 300 to enable the activation of the sensors appropriately beforehand, and block 302 may be modified to perform the comparison accordingly. For example, if it take two beacon intervals to activate the sensor(s), block 302 may be modified to determine whether there are X beacon frame count values to the beacon frame triggering the measurements, wherein X is a number greater than or equal to two.

Because of the unreliability of the radio channel or other conditions, the sensor device may sometimes miss detection of the beacon signal. For such occasions, the sensor device may enable a back-up mechanism to still be capable of starting the measurements on time. Figure 4 illustrates such an embodiment.

Blocks denoted by the same reference numbers as in Figure 3 represent the same or substantially similar functions or operations. Referring to Figure 4, the sensor device may, upon receiving the message indicating a frame count value triggering the start of the measurements, determine the timing of the beacon frame carrying the frame count value in terms of an internal clock of the sensor device (block 400).

The sensor device has all the information needed to determine the timing of the coming beacon frame that will trigger the measurements and, as a consequence, the sensor device may use its internal clock as a back-up for triggering the measurements. For example, if the beacon periodicity is P time units of the internal clock, the latest beacon having a frame count value N+1 has been received at time T of the internal clock, and the frame count value triggering the measurements is o N+3 (as in Figure 2), the measurements shall start at a time instant T+2*P of the AN internal clock. The sensor device may, when standing by for the beacon frames, 5 monitor the internal clock for the time instant T+2*P. Upon detecting the beacon a frame having the frame count value triggering the measurements in block 306, the A 30 process may proceed to block 308 in the above-described manner. In case the E beacon is not detected, the internal clock may be monitored in block 402. Upon o detecting that the internal clock value corresponding to the determined detection S time of the beacon frame triggering the start of the measurements has expired and N no beacon frame has been detected, the process may proceed from block 402 to N 35 —block308 to start the measurements. As a consequence, the measurements may be started in time even in a case where the triggering beacon frame is not detected.

In an embodiment, the sensor device starts acquisition of the measurement data before said detection of the specific frame count value in a received beacon signal and adds, upon detecting the specific frame count value in a received beacon signal, a time stamp to the measurement data to indicate said start of the measurements.

Figure 5 illustrates such an embodiment.

Referring to Figure 5, instead of activating the sensors to stand by for the start of the measurements, the sensor device may start the measurements before detecting the beacon frame triggering the measurements.

In the embodiment of Figure 5, the measurements are started during the guard time of the beacon frame expected to = have the frame count value triggering the measurements or even a beacon frame preceding the beacon frame expected to have the frame count value triggering the measurements.

In other words, the sensor device acquires and stores measurement data during the guard time.

Upon detecting in block 302 that the next beacon frame will trigger the start of the measurements, the sensor device may — configure its sensor(s) to start measurements beforehand during the guard time (block 500). When the beacon frame having the frame count value triggering the start of the measurements is detected in block 306, the sensor device marks the stored measurement data with a label indicating the start time in the measurement data.

The sensor device may store measurement time information in connection — with the measurement data to indicate for each measurement data sample the measurement timing in terms of the internal clock of the sensor device.

Upon detecting the beacon frame triggering the start of the measurements, the sensor device may acquire the value of the internal clock at the detection time.

Then, the sensor device may search the measurement data sequence for a sample associated — with the same value of the internal clock as the measurement timing and label the o sample as the starting point of the measurements (block 502). In an embodiment, AN the sensor device cuts out and discards measurement data preceding the starting 5 point (block 504). a With the procedure of Figure 5, any delays between the detection and TY 30 the actual start of the measurements can be reduced.

E Figure 6 illustrates an embodiment that is a combination of the o procedures of Figure 4 and 5. Accordingly, the sensor device may start the S measurements beforehand according to Figure 5 and, additionally, determine the N upcoming reception time of the beacon frame triggering the start of the N 35 measurements, the reception time being determined in terms of the time measured by the internal clock of the sensor device.

Upon detecting the beacon frame triggering the start of the measurements in block 306, the procedure may follow the process of Figure 5 and the start time may be labelled to the measurement data.

Optionally, the measurement data preceding the start time may be cut (block 504). However, upon missing the detection of the beacon frame triggering the start of the measurements, the procedure may follow the process of Figure 4. In other words, upon detecting the expiry of the expected detection time of the beacon frame in block 402, the sensor device may label the expected detection time as the start time of the measurements.

The procedure of Figure 6 improves the procedure of Figure 4 in the sense that potential delay caused by determining that the beacon has not been detected does not cause missing some of the measurement data.

Since the acquisition of the measurement data has been started beforehand, the measurement data is available, and the sensor device may just determine the position for the label.

The procedure of Figure 6 improves the process of Figure 5 — with the back-up mechanism of the embodiment of Figure 4. Accordingly, there is no need to actually detect the beacon frame in order to start the measurements synchronously with the other sensor devices.

According to an aspect, a procedure for the controller apparatus comprises: broadcasting the periodic beacon signal comprising a field for a frame — count value incrementing with every broadcasted beacon signal; determining a start time of measurements and determining a specific frame count value of the periodic beacon signal corresponding to the start time; transmitting, to a plurality of sensor devices, a message comprising the specific frame count value of the beacon signal for triggering start of measurements.

Figure 7 illustrates an embodiment of such a procedure. o Referring to Figure 7, the controller may determine a start time for AN measurements in block 700. Block 700 may be initiated upon receiving a 5 notification from an application executed in the server computer or in another a device controlling the start and execution of the measurements and, optionally, A 30 analysis of the measurement data.

Since the start of the measurements is E synchronized to the beacon frames, block 700 may include determining the start o time in terms of the frame count value of the beacon frames.

In practice, block 700 S may include selecting a frame count value that will trigger the start of the N measurements.

The selected frame count value may be sufficiently far in the future N 35 to allow the communication of the frame count value to the sensor devices and, in some embodiments to allow the sensor devices to acknowledge the reception of the frame count value.

In block 702, the controller transmits the message(s) indicating the frame count value to the sensor devices. In embodiments where the message(s) comprises a multicast message addressing all the sensor devices, a single message maybe sufficient to deliver the frame count value to the sensor devices. In other embodiments, the controller may transmit multiple (unicast and/or multicast) messages to reach all the sensor devices. In block 704, the controller receives acknowledgment messages from the sensor devices. In block 706, the controller transmits the beacon frame having the specific frame count value that triggers the start of the measurements in the sensor devices. In block 708, the controller may collect the measurement data from the sensor devices and forward the measurement data to the device executing the application, e.g. the server computer. In an embodiment, the application is executed in the controller apparatus. Figures 8 and 9 illustrate embodiments of block diagrams of the — controller apparatus and the sensor device, respectively. Figure 8 illustrates an embodiment of a structure of the above-mentioned functionalities of an apparatus executing the functions of the controller apparatus in the process of Figure 2 or Figure 7 described above. The apparatus may be a wireless device configured to comply with specifications of a wireless network comprising the controller apparatus and the sensor devices 110 to 114. The apparatus may be or may be comprised in a computer, a router device, in one of the sensor devices 110 to 114, or any other apparatus provided with radio communication capability. The apparatus may be an electronic device comprising electronic circuitries for realizing some embodiments of the controller apparatus. Referring to Figure 8, the apparatus may comprise a communication o circuitry 50 providing the apparatus with capability of communicating in at least AN one wireless network. The communication circuitry may comprise at least one 5 processor or a processing circuitry. The communication circuitry may employ a a radio interface providing the apparatus with radio communication capability. The A 30 radio interface may comprise a radio modem 58 and radio frequency (RF) E circuitries 52 providing at least a part of the above-described physical layer(s) of o the wireless device. The radio interface may support frame transmission and S reception according to the principles described above. The RF circuitries 52 may N comprise radio frequency converters and components such as an amplifier, filter, N 35 = and one or more antennas. The radio modem 58 may comprise baseband signal processing circuitries such as (de)modulator and encoder/decoder circuitries.

In embodiments where the apparatus executes the measurement application, the apparatus may further comprise an application processor 56 executing one or more computer program applications that receive an input from an operator to perform the measurements and to analyse the measurement data.

The application processor may form an application layer of the controller apparatus.

The application processor may execute computer programs forming a primary function of the apparatus, e.g. to execute one or more signal processing applications processing the measurement data acquired from the sensor devices.

The apparatus may further comprise a memory 60 storing one or more computer program products 62 configuring the operation of said processor(s) of the apparatus.

The memory 60 may further store a configuration database 64 storing operational configurations of the apparatus.

The configuration database 64 may store, for example, the measurement configurations.

The memory may further a database 66 for the measurement data and/or results of the measurement data — analysis.

The communication controller may comprise a measurement controller 54 configured to carry out block 700, for example.

Upon determining the measurement start time, the measurement controller may indicate the start time to a frame processor 56 that may be configured, in response thereto, to generate the message indicating comprising a frame count value of a beacon frame that will trigger the start of the measurements.

The frame processor may then perform block 702 or 203. The frame processor may also control the periodic beacon transmissions.

The frame processor may generate contents to the frames to be transmitted and output the frames to the radio modem 58 for transmission.

The — frame processor may also extract the contents of the received frames, e.g. extract o measurement data and acknowledgment messages in the above-described manner.

AN Figure 9 illustrates an embodiment of a structure of the above- 5 mentioned functionalities of an apparatus for the sensor device executing the a process of Figure 2 or any one of the embodiments of Figures 3 to 6 described A 30 above.

The apparatus may be the sensor device, or the apparatus may be E controlling a state-of-the-art sensor device, e.g. an accelerometer, an optical sensor, o or another sensor configured to measure an object.

The apparatus may comply S with the specifications of the wireless network comprising the controller N apparatus.

The apparatus may be an electronic device comprising electronic N 35 — circuitries for realizing some embodiments of the sensor device with the radio communication capability.

Referring to Figure 9, the apparatus may comprise at least one processor 10 controlling the operations of the sensor device, e.g. the measurements and the communication with the controller apparatus.

The processor(s) 10 may employ a radio interface providing the apparatus with radio communication capability.

The radio interface may comprise a radio modem 18 and radio frequency (RF) circuitries 12 providing at least a part of physical layer(s) of the sensor device for radio communications.

The radio interface may support frame transmission and reception according to the principles described above.

The RF circuitries 12 may comprise radio frequency converters and components such as an amplifier, filter, and one or more antennas.

The radio modem 18 may comprise baseband signal processing circuitries such as (de)modulator and encoder/decoder circuitries.

The apparatus may further comprise a frame processor 12 configured to generate frames for transmission and to extract contents of received frames, e.g. the beacon frames and the message carrying the frame count value of an upcoming beacon frame triggering the start of the measurements.

The apparatus may further comprise a memory 20 storing one or more computer program products 22 configuring the operation of said processor(s) of the apparatus.

The memory 20 may further store a configuration database 24 storing operational configurations of the apparatus.

The configuration database 24 may store, for example, the measurement configurations.

The memory 60 may further store a buffer 26 for the measurement data to be transmitted.

The apparatus may further comprise one or more sensor heads 30 comprising physical sensing components, such as an optical sensor head and/or a motion sensor head (e.g. an accelerometer). In such embodiments, the sensor o heads may be integral parts of the apparatus.

In other embodiments, the apparatus N is external to the sensor heads 30 or state-of-the-art sensor devices and connected 5 or coupled to the sensor heads/devices 30. The apparatus may then control the a operation of the external sensor(s) according to the principles described above.

A 30 The processor(s) 10 may comprise a measurement controller 13 E configured to control the activation and operation of the sensor head(s), as o described in the embodiments above.

The measurement controller may comprise S a measurement data processor 19 configured to process the measurement data N received from the sensor head(s) 30 into a form suitable for transmission.

The N 35 processing may include filtering the measurement data, computing one or more parameters from the measurement data, etc.

As used in this application, the term ‘circuitry’ refers to one or more of the following: (a) hardware-only circuit implementations such as implementations in only analog and/or digital circuitry; (b) combinations of circuits and software and/or firmware, such as (as applicable): (i) a combination of processor(s) or processor cores; or (ii) portions of processor(s)/software including digital signal processor(s), software, and at least one memory that work together to cause an apparatus to perform specific functions; and (c) circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present. This definition of ‘circuitry’ applies to uses of this term in this application. As a further example, as used in this application, the term “circuitry” would also cover an implementation of merely a processor (or multiple processors) or portion of a processor, e.g. one core of a multi-core processor, and its (or their) accompanying software and/or firmware. The term “circuitry” would also cover, for example and if applicable to the particular element, a baseband integrated circuit, an application-specific integrated circuit (ASIC), and/or a field- programmable grid array (FPGA) circuit for the apparatus according to an embodiment of the invention. The processes or methods described in Figures 2 to 7 may also be carried out in the form of one or more computer processes defined by one or more computer programs. A separate computer program may be provided in one or more apparatuses that execute functions of the processes described in connection with the Figures. The computer program(s) may be in source code form, object code form, or in some intermediate form, and it may be stored in some sort of — carrier, which may be any entity or device capable of carrying the program. Such o carriers include transitory and/or non-transitory computer media, e.g. a record AN medium, computer memory, read-only memory, electrical carrier signal, 5 telecommunications signal, and software distribution package. Depending on the a processing power needed, the computer program may be executed in a single A 30 electronic digital processing unit or it may be distributed amongst a number of E processing units.

o Embodiments described herein are applicable to wireless networks S defined above but also to other wireless networks. The protocols used, the N specifications of the wireless networks and their network elements develop N 35 rapidly. Such development may require extra changes to the described embodiments. Therefore, all words and expressions should be interpreted broadly and they are intended to illustrate, not to restrict, the embodiment. It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. Embodiments are not limited to the examples described above but may vary within the scope of the claims.

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Claims (15)

1. A method for synchronizing sensor devices, comprising: broadcasting (200, 202, 204, 206), by a controller, a periodic beacon signal comprising a field for a frame count value; transmitting (203), by the controller to a plurality of sensor devices, a message comprising a specific frame count value for triggering start of measurements; upon detecting, by the plurality of sensor devices, the specific frame count value in areceived beacon signal, triggering (208) the start of said measurements.

2. The method of claim 1, wherein the frame count value increments as the periodic beacon signals are broadcasted.

3. The method of claim 1 or 2, wherein the sensor devices acknowledge reception of the message by transmitting (210, 212, 214) an acknowledgment message to the controller, and wherein a time interval between the transmission of the message and transmission of the beacon signal having the specific frame count value is sufficiently high to enable said transmission of the acknowledgment message from the sensor devices.

4. The method of any preceding claim, further comprising as performed by at least one of the plurality of sensor devices: determining (400), beforehand on the basis of periodicity of the beacon signal and o the specific frame count value, a clock value of an internal clock of the at least one O of the plurality of sensor devices when receiving the beacon signal having the O specific frame count value; © _ 30 upon determining (402) that the beacon signal having the specific frame count = value has not been detected upon expiry of the clock value, triggering (308) the 2 start of said measurements.

2 ä 5. The method of any preceding claim, wherein the plurality of sensor — devices start (500) acguisition of measurement data before said detection of the specific frame count value in a received beacon signal and add, upon detecting the specific frame count value in a received beacon signal, a time stamp to the measurement data to indicate said start of the measurements.

6. The method of claim 5, wherein the plurality of sensor devices start the acquisition of measurement data during a guard time preceding the transmission of the beacon signal having the specific frame count value.

7. The method of any preceding claim, wherein the message is a unicast message or a multicast message.

8. A controller apparatus (120) for a sensor system, comprising means for performing: broadcasting a periodic beacon signal comprising a field for a frame count value; determining a start time of measurements and determining a specific frame count value of the periodic beacon signal corresponding to the start time; transmitting, to a plurality of sensor devices (110, 112, 114), a message comprising the specific frame count value of the beacon signal for triggering start of measurements.

9. The controller apparatus of claim 8, wherein the means are configured to receive, from the plurality of sensor devices, acknowledgement messages acknowledging reception of the message, and wherein a time interval between the transmission of the message and transmission of the beacon signal having the o specific frame count value is sufficiently high to enable transmission of the AN acknowledgment messages.

N

O a

10. The apparatus of claim 8 or 9, wherein the means are configured to A 30 increment the frame count value as the periodic beacon signals are broadcasted. x a o

11. An apparatus for a sensor device (110, 112, 114) of a sensor system,

N 3 comprising means for performing: N receiving, from a controller (120), a periodic beacon signal comprising a field for a N 35 frame count value;

receiving, from the controller, a message comprising a specific frame count value for triggering start of measurements; upon detecting the specific frame count value in a received beacon signal, triggering the sensor device to the start said measurements.

12. The apparatus of claim 11, wherein the means are configured to acknowledge reception of the message by transmitting an acknowledgment message to the controller, and wherein a time interval between the transmission = of the message and transmission of the beacon signal having the specific frame count value is sufficiently high to enable said transmission of the acknowledgment message.

13. Theapparatus of any preceding claim 11 or 12, wherein the means are — further configured to: determine, beforehand on the basis of periodicity of the beacon signal and the specific frame count value, a clock value of an internal clock of the sensor device when receiving the beacon signal having the specific frame count value; upon determining that the beacon signal having the specific frame count value has not been detected upon expiry of the clock value, trigger the start of said measurements.

14. The apparatus of any preceding claim 11 to 13, wherein the means are o configured to start acguisition of measurement data before said detection of the AN specific frame count value in a received beacon signal and to add, upon detecting 5 the specific frame count value in a received beacon signal, a time stamp to the a measurement data to indicate said start of the measurements. A 30 E

15. The apparatus of claim 14, wherein the means are configured to start 9 the acguisition of measurement data during a guard time preceding the 2 transmission of the beacon signal having the specific frame count value. ä