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Synchronization Explained

Xsens DOT App allows time synchronization among all sensors connected, resulting in highly accurate time-synchronized data: 1 part per million (ppm) difference is expected after 30 minutes (1.8ms)  and 4ppm (10ms) after 60 minutes. This is achieved by selecting one of the sensors as root node and all the others as scanners. The scanners will adjust their timestamp in order to match the one from the root node.

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As a result, the csv file will show identical timestamp values for each sensor. Since the start command sent via Bluetooth has different delays between each sensor, sensors might show several extra data packets that need to be taken into account when analysing data. For simplicity, extra packets can be removed during post processing.


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Figure 1: CSV file when synchronization is off. Timestamp values do not match.

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Figure 2: Synchronization on. Timestamp values from different sensors match. DOT 1 shows one extra packet at the beginning of the logging.

 

Synchronization requires 8 seconds of data acquisition before the beginning of the measurement. And the sensors will disconnect and re-connect to the mobile phone during synchronization. This phase is extremely important to ensure a successful synchronization.  In order to increase its success rate, it’s advised to keep the all the sensors and the mobile phone as close as possible, no more than 50cm apart from each other during the synchronization period. As a useful practice, sensors can be placed on a flat surface with the phone close to them to ensure a successful synchronization.

 

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Other useful tips are:

  • Avoid stacking sensors on top of each other to prevent battery interference.
  • In a scenario where the sensors need to be placed on a subject for measurement (i.e. gait analysis) it is recommended performing the synchronization first with the sensors on a table and afterwards placing them on the body.
  • Synchronization might be unsuccessful in case of an environment with strong interferences from other devices (i.e. conferences or trade shows). In this case, it is recommended not enabling synchronization.
  • When using only one sensor synchronization will not have any effect on the data. Therefore, in this scenario, it is recommended disabling synchronization.

Advantages of enabling synchronization include:

  • Easier comparison between multiple sensors
  • More accurate time reference

Disadvantages of enabling synchronization include:

  • Increased time to begin recording. Synchronization requires 8 seconds of data acquisition before starting to record.
  • Higher chances of failure

In Xsens DOT App version 2020.2.2, the synchronization can be enabled or disabled according to the user preference in both Real-time streaming and Recording mode. Find here a guide to Xsens DOT App for additional information. Additionally, keep in mind that the outcome of the synchronization highly depends on the mobile phone used; different brands have different success rates for synchronization. If your phone does not allow a successful synchronization, it is recommended disabling synchronization or utilizing a different mobile phone. You can check out our supported devices in our release notes.

We are working on improving the robustness of synchronization by improving the successful rate throughout different platforms, as well as optimizing the working process in the next release.

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