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Sensor internal timestamp and General Queries

JenniferJennifer
in Requests & Bug Reports
Hi,

While using the MetaMotion R sensor I came across the following queries:

The .csv file of data obtained from the sensor includes a timestamp. From previous forum posts this timestamp appears to be that of the phone once the data is received and not of the sensor. What is the sampling rate of the sensors in order to see if there is any drift? Is it fixed internally?

Also while using 'Log', from previous posts you had mentioned that using the timestamps from the .csv is accurate enough. While using 'stream' however (at a rate of 50Hz for example), since bluetooth receives the data in packets of three, should the the timestamps be adjusted manually to read 0.02s, 0.04s, 0.06s? Would this be accurate enough and match with the sensor timestamp? And is there a way to obtain the sensor's timestamp?

Some other general questions of use: 
- If the sensors go to sleep after 20 minutes, are they able to wake up automatically after activity starts again?
- With regards to the Android app, is there a way to connect more than 1 sensor at a time using 'stream' function?

Many thanks in advance,

Best,

Comments

  • EricEric
    edited October 2017
    1. Sensor sampling rate depends on what you set it to.  You should be configuring the sensors (setting desired data rate, range, etc) before collecting data.

    2. Reconstructing timestamps is discussed in this thread here:
      https://mbientlab.com/community/discussion/1934/metahub-timestamps#latest

      Alternatively, you can allocate part of the BLE packet to include a reference point from the firmware which the SDKs use to compute the timestamps, similar to how the logged data works.  That is detailed in the documentation
      https://mbientlab.com/androiddocs/latest/data_route.html#account

    3. Sensors do not go to sleep on their own.

    4. I'm not sure what you are asking with regards to connecting more than 1 sensor with the `stream` function.
  • JenniferJennifer
    Hi Eric 

    Many thanks for your prompt reply! 

    Using the Android app, I am not able to connect two-four sensors and stream data from them. Is this a limitation using Android so would using a Windows 10 PC solve this issue?

    With regards to the timestamps; when streaming data, due to bluetooth the firmware sends three data samples per packet, so the timestamp is repeated three times. I have read the thread below which states that a simple reconstruction of the timestamp could be made from the sampling frequency selected. 

    My question is, the sensors produce the data at accurate intervals, is it accurate to the extent of it being exactly equal to the sampling rate selected? I am simply asking this question to determine whether there is any time drift if the sensors are used for 3 hours or longer.

    Many thanks!
  • EricEric
    The MetaWear companion app is only designed to connect to one board at a time.  Use MetaBase to stream from 2 devices simultaneously.

    You won't be able to use more than 2 sensors at full bandwidth simultaneously so you'll have to pick up another Android device or reduce the sampling frequency across all the boards.

    The sensors have 1% error in the data rate so not all of the data will be perfectly spaced.
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