Confusion about accelerometers

[espruino-discuss2]

Posted at 2023-08-08 by KTibow

From my understanding an object at rest should be almost 0 m/s2. Why does my Bangle.js 2 report a minimum acceleration of 1 m/s2 then? Here's a reading while it's still:

{ x: -0.02648925781, y: 0.080078125, z: -1.01220703125, mag: 1.01571514756, diff: 0.00561523437 }

I've been spending a while asking GPT and Googling to no avail. Is this accelerometer broken? Is this intended behavior? What's going on?

[espruino-discuss2]

Posted at 2023-08-08 by KTibow

I now realize I was a bit confused because 1g = 9.8 m/s2. Still - I'm confused if this is the deal with all accelerometers.

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Posted at 2023-08-08 by @thyttan

Wikipedia first paragraph on accelerometers:

[...] an accelerometer at rest on the surface of the Earth will measure an acceleration due to Earth's gravity, straight upwards[3] (by definition) of g ≈ 9.81 m/s2. By contrast, accelerometers in free fall (falling toward the center of the Earth at a rate of about 9.81 m/s2) will measure zero.

https://en.m.wikipedia.org/wiki/Accelerometer

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Posted at 2023-08-08 by KTibow

Thanks. Are there any apps that implement a conversion to coordinate acceleration I could use as a reference?

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Posted at 2023-08-09 by @gfwilliams

This has been asked a few times - including in a forum message that got surfaced today! https://forum.espruino.com/conversations/386551/#16980121

Basically it's tricky because to really know you need to take account of twist, and we don't have a gyro. But because we know that when the total G=1g we're stationary, we can use that to set a baseline - code in that link above