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llau.1
Visitor II
July 10, 2020
Question

Question about the sign of raw sensor measurement

  • July 10, 2020
  • 1 reply
  • 724 views

I have a problem when i read the AN4508 application notes.

In the figure shown below,

0693W000001sakMQAQ.png

Xb, Yb and Zb are device body axes with a forward-right-down configuration. 

XA, YA and ZA are the accelerometer sensing axes.

When sensor is placed still on the table, i expected the value from accelerometer z axis is -1g because there is a downward acceleration due to gravity.

0693W000001sakvQAA.jpg

But the application notes mentions that Az would output +1g when Zb pointing down .

0693W000001salAQAQ.png

Why the signed is reversed?

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    1 reply

    E
    Eleon BORLINI
    ST Employee
    July 16, 2020

    Hi @llau.1​ , the axis direction of the sensor is related to the fact that the acceletometer is sensing the constraint reaction of the table top on which you put the accelerometer itself. For this reason, if you place an accelerometer on a table with the top marking above (Za axis), you will see a +1g response from the sensor, even if the Z axis is typically defined pointing towards the floor (Zb). In any case, if you have to design an application mounting an accelerometer, please refer to the datasheet of that specific accelerometer (for example, below it the LIS2DW12 case with positive axis directions). Regards

    0693W000001t0lzQAA.png

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