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CC2650STK: Accelerometer orientation

Part Number: CC2650STK
Other Parts Discussed in Thread: CC2650

Hi E2E,

Is there a proper orientation required for the device to use the accelerometer properly? When the customer is using the device where the device logo on the accelerometer is facing up, all z-axis measurements are negative. When the logo on the accelerometer is facing down, all the z-axis measurements are positive. Also, when the sensortag is connected to the app, according to the customer, the z-value seems to read properly for a couple seconds, and then the reading displays as 4.0 or -4.0 consistently for a longer period of time. Is the accelerometer working properly? Is calibration for the accelerometer needed and how they can do this?

Here are some screenshots.  The sensortag was at rest on the table.

Regards,
Carlo

  • Carlo,

    This is likely due a bug in the app or inaccuracies with the accelerometer. Can you try debugging the SensorTag and see if the issues still persist?

    Regards,

    Daniel

  • Hi Daniel,

    Thanks for the advice.  Since the sensortag mentioned above may have inaccuracies, I have a second C2650 sensortag that I am using now and the z-axis is not at zero when I start but is at 1.0.  Can you help me calibrate the Z-axis to 0 when the sensortag is sitting flat on the desk?

    I have a devpack, but since I am not an engineer or a coder, I do not know how to code the debug to calibrate the z-axis measurement to 0. I need a simple accelerometer to use for measuring Z-axis vibration for a research project I am working on that will benefit patients. I am a healthcare professional.  Would you be able to send me step by step instructions on how to code to calibrate the z-axis to 0?

    Your help is greatly appreciated.

    thanks,

    Cathy

  • Hi Carlo, could you please let me know if I sent my reply properly to daniel? Or did it send to me?

    Thanks,

    Cathy

  • Cathy,

    I believe that if the CC2650STK is flat and stationary on a table it should read 1G in the Z-axis since it is experiencing a force down (Z direction) of 1.0 Gs. It would read 0.0 G if it were not experiencing any forces from gravity.

    Can you also verify in the app that the SensorTag is running the latest firmware? If not, you can update it using the mobile app.

    Regards,

    Daniel

  • Hi Daniel,

    You're right about the down force of gravity, which I did not think about. I need to measure Z-axis vibration on the surface of a piece of machinery. 

    To clarify, if I want to truly capture the Z-axis vibration that the machine is producing, do I "zero" the 1.0 G and subtract 1.0 from all measured Z-values of the machinery?

    thanks for your help,

    Cathy

  • Cathy,

    There isn't a way to zero out the Z-axis without changing the code and reprogramming the SensorTag, so you will need to compensate for that in your measurements. 

    Daniel

  • Hi Daniel

    Thanks for your help!

    The CC2650 sensor tag measures acceleration in G units. 1.0 G unit being the force of gravity, as you have mentioned.  If I wanted to measure vibration acceleration in the Z-axis, and convert to m/s^2 as per ISO 2631-1 (I can find a unit converter online), do I then NOT include the 1.0 G when converting to m/s^2 and only count Z-axis measurement above 1.0 G because we are always experiencing gravity.

    For example, if the sensor tag measured z-axis acceleration at 1.3 G, to convert to vibration acceleration m/s^2, do I only convert the 0.3 and not include the 1.0 because we are always experiencing the force of gravity?

    1.3 G converted to m/s^2 is a much much larger value than 0.3 G converted to m/s^2.

    appreciate the consult,

    Cathy

  • Cathy,

    This is beyond the scope of my knowledge and beyond the scope of this forum.

    The SensorTag is intended to be a way for customers to evaluate the CC2650 Wireless MCU and to serve as a reference for custom designs. The accelerometer / gyroscope on the EVM is part number MPU-9250, so you may want to reach out to the manufacturer with specific questions.

    If you have any questions about the CC2650 wireless MCU or need help with software/ designing with TI parts, please let me know! :) 

    Regards,

    Daniel

  • Ok thank you,

    All the best!

    Cathy