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BNO085 Accelerometer Scale Factor
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BNO085 Accelerometer Scale Factor

by demir on Wed Jun 02, 2021 3:59 pm

What is the scale factor for Accelerometer?

I received raw accelerometer data but how can convert to g?

I received mostly -160,80,4032 accelerometer data as x,y,z when staying stable position; but how can convert it to calculated accelerometer data?

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Re: BNO085 Accelerometer Scale Factor

by millercommamatt on Tue Jun 08, 2021 3:44 pm

Which library are you using and it there a reason you are raw ADC values and not the translated values the Adafruit libraries provide?

An you using specific example code? If so, what?

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Re: BNO085 Accelerometer Scale Factor

by gammaburst on Tue Jun 08, 2021 8:03 pm

demir's question was cross-posted in this discussion:
viewtopic.php?f=19&t=179820&p=875668#p875668

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Re: BNO085 Accelerometer Scale Factor

by sj_remington on Thu Jun 10, 2021 5:12 pm

If the accelerometer is held still, with any axis perfectly vertical, the acceleration is exactly +/- 1 g along that axis.

Use that fact and the raw data to figure out the scale factor and offset for each axis.

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Re: BNO085 Accelerometer Scale Factor

by gammaburst on Fri Jun 11, 2021 3:08 am

No, gravity varies by about 0.7% due to Earth's irregular shape and composition.
Also, Earth's rotation reduces your apparent gravity as you approach the equator.
The Sun's and Moon's positions also have a slight effect (think tides).

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Re: BNO085 Accelerometer Scale Factor

by sj_remington on Fri Jun 11, 2021 11:03 am

Correct, but unfortunately, the BNO085 cannot detect those slight variations.

Which is why most people assume exactly 1 g for the purpose of computing MEMS accelerometer scale and offset factors.

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Re: BNO085 Accelerometer Scale Factor

by gammaburst on Fri Jun 11, 2021 1:13 pm

The BNO085 (and other similar inexpensive sensors) can easily detect variations of 0.7% of a g.

Take care using rigid words such as "exact", "cannot", and "fact".

A 0.7% error is acceptable for many applications, so your suggested inversion method of calibration would be adequate, and it's easy to do.

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Re: BNO085 Accelerometer Scale Factor

by sj_remington on Fri Jun 11, 2021 1:37 pm

Agreed!

When using the BNO085 to measure g at the North or South pole and at the top of Mt. Everest, one would certainly want to take those calibration considerations under advisement.

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