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Not loging correctly

narrinarri
in Android
I am ussing the metabase android app in a nexus tablet and when i am loging the data I had the following elapsed times at 50 Hz which is nearly perfect 

image0   0.021     0.041     0.062     0.081     0.101  0.122    0.141    0.161    0.182   0.201     0.221

but when i go to 800Hz I have this:

0    0.002        0.003      0.005    0.006   0.008   0.009     0.011

which is very different, also i tried another time and i have the following at 100Hz:

0   0    0    0.033    0.033    0.033    0.067     0.067     0.067      0.089    0.089     0.089   0.123

which is as it has taken three measures at a frequency of 33Hz

Comments

  • narrinarri
    I am ussing the metabase android app in a nexus tablet and when i am loging the data I had the following elapsed times at 800 Hz 

    image
    0
    0
    0.00146484375
    0.0029296875
    0.00439453125
    0.005859375
    0.00732421875

    which makes that the frequency is similar but not the one selected. how can i put an exact frequency?Is it problem of the app or of the Bluetooth transmission? Can it be because i am logging data simultaneously from both sensors?


  • EricEric
    edited August 2017
    Are you logging or streaming?  It sounds like you are logging 50Hz then 800Hz, and streaming at 100Hz at a different time.

    Also, which board are you using?
  • narrinarri
    edited August 2017
    I will try to clarify it @Eric.

    I am taking data of the accelerometer and the Gyroscope, both having the same frequency configuration to take data, for that i am using theMetaMotianR board, the application using to take the data and see it is the MetaBase app, in a nexus7 tablet running Android 6.0, I also have acces to an Iphone 6 device with the same app runing IOS 10.

    When I take the data, I select the log option and my device, then I activate the accelerometer and the gyroscope at the same frequency and take the data. 

    When I have the data in my computer, I analize it with a matlab script that take the elapsed time and substract it from the previous one, having the difference between the two data. 

    The problem comes when I realize that the times are not the exact 1/frequency, while the mean is quite similar. Being it as follows:

    for data at suposely 25Hz (0,04s):

    mean: 0.04011171 Variance 0.00000051

    Some time-differences (I can´t put all of them because there are millions of them):

    0.03955078    0.03955078     0.04040587  0.04101563    0.04101563 


    My question is the following:
    Is there any setting in the apk that makes a condition for having the exact time(1/frequency), or is it always an aproximate time?.
    Also the problem can be due to the bluetooth transmission or the usage of both sensors at the same time?
  • EricEric
    You will never have all of the data be equally spaced at the set ODR.  Furthermore, the few time differences you have posted, and I assume this is true of the other values as well, are approximately +/-1% of 0.04s, which is expected.


  • narrinarri
    Thanks @Eric for the response

    So there is no way with the equipment I have to have it equally spaced? or either with another equipment?

    Also what is the reason for that (the apk, bluetooth conection, device programming)?
  • EricEric
    No, you will never have all data at equally spaced time intervals from the boards.

    The IMU itself does not always produce data at the exact timing and the firmware has to mark each data sample with a value used by the SDKs to calculate the actual time.
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