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CC2650: Acceleration Measurement Capture

ArTzy
Mastermind 19220 points
Part Number: CC2650
Other Parts Discussed in Thread: CC2640R2F, CC2642R

Hi Experts,

Good day.

Customer is working on a research project and would like to verify a few details about acceleration measurement/capture using the sensor tag CC2650. They are working on a healthcare research project and had a few questions re: sensor tag CC2650 and the accelerometer.

[1] Does the sensor tag automatically detect frequencies of 0.5 to 80 Hz (we're referring to an ISO guideline re: accelerometer use), or do we have to intentionally set it to detect this range?

[2] In terms of vibration/acceleration data sampling frequency, there is an option in the settings to set how many data points per second are recorded/displayed. Customer chose 3 data points per second for their data collection. Does this literally mean the sensor tag only sampled 3 data points per second or were they "mean" values collected 3 times per second?

Someone has mentioned that for ISO standards we should have a data sampling of at least 50-100 points per second. Please clarify if that is recommended when using the sensor tag CC2650. Thanks

Regards,
Archie A.

  • 0
    TI__Mastermind 42870 points

    Hi Archie,

    1. Are you referring to how fast the accelerometer is able to provide data? If so, then I believe this information should be available in the accelerometer's datasheet. 

    2. The accelerator used in the sensor tag is the MPU-9250, so I would check the datasheet for that device to see if it provides this information somehow. If it does not, then you may be able to implement this in the software by adding a counter to the interrupt associated with the accelerometer and having a timer to calculate the throughput. 

    3. The recommended data sampling rate will be application-dependent. Something to keep in mind would be that in BLE the connection interval will be the max possible latency you experience, so depending on how many data points you want to send over BLE and how fast you will need to adjust the connection interval. 

    Best Regards,

    Jan

  • 0
    Guru 86765 points

    Hi ArTzy,

    Accelerometer sampling rate is done internally usually in the range from 1 Hz to KHz range. You need to write to accelerometer register to set this.

    I recommend your customer use the launchpad version, because you can connect any accelerometer board to it. I recommend the CC2640R2F Launchpad or CC26x2R1 Launchpad.

    -kel

  • 0 in reply to Jan
    TI__Mastermind 19220 points

    Hi Experts,

    Thank you.

    Jan said:
    2. The accelerator used in the sensor tag is the MPU-9250, so I would check the datasheet for that device to see if it provides this information somehow. If it does not, then you may be able to implement this in the software by adding a counter to the interrupt associated with the accelerometer and having a timer to calculate the throughput.

    Could you please highlight where in the data sheet (page number and paragraph) I can find the information about the MPU-9250 sampling frequency? We did not use software to set a counter for sampling frequency, we used the MPU-9250 "as is" connected to an apple iPad to collect vibration data.

    Jan said:
    1. Are you referring to how fast the accelerometer is able to provide data? If so, then I believe this information should be available in the accelerometer's datasheet.

    What we mean is does the sensor tag CC2650 automatically capture vibrations in the 0.5 to 80 Hz ranges? Or do we have to set it to do so? Would like to clarify that we are talking about the sensitivity of the accelerometer to pick up vibrations of certain "sizes/frequencies", not the sampling frequency per second.

    Regards,
    Archie A.

  • 0 in reply to ArTzy
    TI__Mastermind 42870 points

    Hi Archie,

    I have found the register map and the datasheet at the following links:

    https://invensense.tdk.com/download-pdf/mpu-9250-datasheet/

    https://invensense.tdk.com/download-pdf/mpu-9250-register-map/

    Best Regards,

    Jan

  • 0 in reply to Jan
    TI__Mastermind 19220 points

    Hello Jan,

    Thank you for responding.

    From your previous notes, I have this response:

    Apology but we have limited information about vibration/acceleration data sampling.

    Thank you.

    Regards,
    Archie A.

  • 0 in reply to ArTzy
    TI__Mastermind 42870 points

    Hi Archie,

    My apologies, looking through the data sheet, looking at table Table 2, it seems the output data rate can go from 0.24Hz to 500Hz when in low power mode and from 4Hz to 4000Hz when in Low noise (active) mode. Is this what you are looking for?

    Best Regards,

    Jan

  • 0 in reply to Jan
    TI__Mastermind 19220 points

    Hello Jan,

    Thank you. The customer needed more time to respond:

    I’m attaching 2 screenshots. I hope this helps with clarifying my questions. Thank you for understanding that I am a healthcare professional and not an engineer.

    For the file name Sensor Tag Image Calibrate, I have a screenshot of calibrating the sampling rate at 0.3 seconds, is this similar to sampling data at 3 times per second? Is each data point samples a “mean” acceleration at that time period, or each data point sampled a discrete measurement of acceleration at that point in time?

    For the file name Sensory Tag Image Acceleration, what is the value of time for each small square on the horizontal axis?  Is each square simply 0.3 seconds because that is what I calibrated the sampling at?  The overall time duration (start time at bottom left and end time at bottom right) is not correct and represents the entire time the sensor tag was on, not the recording time while I was moving the sensor tag.

    Lastly, if I did not make any sensitivity adjustment to the sensor tag, does this model pick up 0.5 to 80 Hz range of vibration out of the box?

    Regards,
    Archie A.


  • 0 in reply to ArTzy
    Guru 86765 points

    Hi ArTzy,

    I reviewed the CC2650 Sensortag MPU-9250 code. The accelerometer sensitivity is set to +-8g and Output Data Rate(ODR) 184 Hz.

    There are generally 2 states the MPU-9250 goes into which are INACTIVE and ACTIVE states.

    INACTIVE state:

    • Waits for movement greater or equal to 8mg.
    • If movement is detected goes to ACTIVE state.

    ACTIVE state:

    • Reads the accelerometer registers at set interval. Interval set by app from 0.1 sec to 2.5 secs.
    • If there is no activity goes to INACTIVE state.

    Using formula t = 1/f, 0.5 Hz is 2 seconds and 80 Hz is 0.0125 seconds. Meaning it can pick up vibration of 0.5 Hz but not 80 Hz.

    The default out of box program will not work for the intended application. But with proper configuration of firmware it can be done. However, since the firmware will be customized, sensortag app can't be used to view the data.

    Regarding hardware BLE MCU CC2650 is old. I recommend CC2640R2F. If PCB space is not a concern CC2642R.

    Regarding MPU-9250 it has gyroscope, accelerometer, magnetometer sensors in 1 IC package. If accelerometer is only needed then choose a IC that only has accelerometer sensor.

    I recently worked in the healthcare industry may I ask the intended application?

    If your customer would be interested in my expert help outside the forum, they can reach out to me through my email at my TI E2E profile.

    -kel